Study of Non-Isothermal Crystallization of Polydioxanone and Analysis of Morphological Changes Occurring During Heating and Cooling Processes

Article Study of Non-Isothermal Crystallization of Polydioxanone and Analysis of Morphological Changes Occurring during Heating and Cooling Processes

Yolanda Márquez 1,2, Lourdes Franco 1, Pau Turon 2, Juan Carlos Martínez 3 and Jordi Puiggalí 1,*

1 Chemical Engineering Department, Escuela d’Enginyeria Barcelona Est (EEBE), c/ Eduard Maristany 19, Barcelona 08930, Spain; [email protected] (Y.M.); [email protected] (L.F.) 2 B. Braun Surgical S.A., Carretera de Terrassa 121, Barcelona 08191, Spain; [email protected] 3 ALBA Synchrotron Light Facility, Ctra BP 1413 km 3.3, Cerdanyola del Vallès, Barcelona 08290, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-934-016684

Academic Editor: Naozumi Teramoto Received: 11 August 2016; Accepted: 19 September 2016; Published: 28 September 2016

Abstract: Non-isothermal crystallization kinetics of polydioxanone (PDO), a polymer with well-established applications as bioabsorbable monofilar suture, was investigated by Avrami, Mo, and isoconversional methodologies. Results showed Avrami exponents appearing in a relatively narrow range (i.e., between 3.76 and 2.77), which suggested a three-dimensional spherulitic growth and instantaneous nucleation at high cooling rates. The nucleation mechanism changed to sporadic at low rates, with both crystallization processes being detected in the differential scanning calorimetry (DSC) cooling traces. Formation of crystals was hindered as the material crystallized because of a decrease in the motion of molecular chains. Two secondary nucleation constants were derived from calorimetric data by applying the methodology proposed by Vyazovkin and Sbirrazzuoli through the estimation of effective activation energies. In fact, typical non-isothermal crystallization analysis based on the determination of crystal growth by optical microscopy allowed secondary nucleation constants of 3.07 × 105 K2 and 1.42 × 105 K2 to be estimated. Microstructure of sutures was characterized by a stacking of lamellae perpendicularly oriented to the fiber axis and the presence of interlamellar and interfibrillar amorphous regions. The latter became enhanced during heating treatments due to loss of partial chain orientation and decrease of electronic density. Degradation under various pH media revealed different macroscopic morphologies and even a distinct evolution of lamellar microstructure during subsequent heating treatments.

Keywords: polydioxanone; surgical suture; non-isothermal crystallization; lamellar stacking; synchrotron radiation

1. Introduction Poly(p-dioxanone) (PDX or PDO) is a synthetic poly(ester-ether) with wide applications in the biomedical field due to its excellent properties (e.g., biodegradability, biocompatibility, bioabsorbability, softness, and flexibility) [1]. In relation to polyglycolide, i.e., the first polyester employed for biomedical applications, the chemical repeat unit of PDO has an ether bond and additional methylene groups that provide greater flexibility. In this way, PDO can be used as a monofilar surgical suture, in contrast with the braided polyglycolide suture. PDO can be completely reabsorbed in a period close to six months, with no significant foreign body reaction being observed in the tissues surrounding the implant.

Polymers 2016, 8, 351; doi:10.3390/polym8100351 www.mdpi.com/journal/polymers Polymers 2016, 8, 351 2 of 18

Several companies have commercialized PDO under different trademarks (e.g., PDS II and MonoPlus™ by Ethicon and B. Braun Surgical S.A., respectively) as a long term surgical suture that appears as an ideal wound support for healing periods longer than four weeks. Other biomedical applications such as bone or tissue fixation devices, fasteners, and drug delivery systems [2] should also be considered since the polymer can be easily injection-molded. Isothermal crystallization of PDO has been extensively studied by different techniques, including polarized optical microscopy and differential scanning calorimetry (DSC) [3–8]. Changes in morphological parameters have also been evaluated by small-angle X-ray scattering [9]. Several works have focused on the influence of degradation on morphology and isothermal crystallization behavior [10–12]. Surprisingly, only a few reports concern the crystalline structure of PDO and lead to some conflicting results. Thus, early studies pointed to an orthorhombic unit cell containing only two molecular segments with a tight pitch (i.e., the chain axis repeat was shortened by 52% compared to the expected value for an extended zig-zag conformation) [13]. Subsequently, an orthorhombic unit cell with space group P212121 and parameters a = 0.970 nm, b = 0.751 nm and c (chain axis) = 0.650 nm was postulated using X-ray and electron diffraction data [14]. The structural model was also supported by quantum mechanical calculations that indicated a chain periodicity given by a single residue with a TGT(-G)TT conformation and a unit cell containing four molecular segments. Crystals obtained from solution had a variable morphology (i.e., the acute apex angle of lozenge crystals varied with the crystallization conditions) that seems a consequence of the peculiar structure of PDO, where different folds for adjacent chain re-entry exist [14]. It is well established that isothermal crystallization of PDO from the melt renders spherulites with a distinctive morphology depending on the crystallization temperature. Specifically, spherulites obtained at high temperature were double-ringed [3] whereas at low temperature they showed a negative birefringence and the typical Maltese cross. Isothermal crystallization kinetic studies linked the different morphologies to two crystallization regimes [3]. Ringed and negative birefringent spherulites were observed by evaporation of concentrated formic acid solutions. It was demonstrated that the a crystallographic axis of constitutive lamellae was oriented along the spherulite radius [14]. The knowledge of non-isothermal crystallization kinetics is essential to achieve the proper microstructure and the required properties of a material. The control of crystallization may reduce undesired effects during processing such as excessive anisotropy of shrinkage, warpage, and insufficient dimensional stability [15]. Nevertheless, it should be considered that non-isothermal crystallizations studies are usually limited to idealized situations, in which external conditions such as the cooling rate is kept constant, although in real situations polymers are cooled down at different rates and with high thermal gradients [16]. Non-isothermal crystallization of PDO has been scarcely studied despite the fact that melt-processed samples (e.g., bioabsorbable surgical sutures) are obtained under non-isothermal conditions. However, some non-isothermal melt crystallization analyses have been performed from DSC data and by applying Ozawa [17] and Cazé [18] methodologies to evaluate Avrami exponents [4]. Values calculated by Ozawa were dependent on the temperature, whereas two different values of the Avrami exponent were determined depending on the cooling rate [6]. Non-isothermal cold crystallization has also been evaluated by calorimetric analysis [19] and Avrami [20,21], Ozawa [17], and Tobin [22] methodologies. In this case, results suggested that the Avrami method was more effective in describing non-isothermal cold crystallization kinetics, with reported values of Avrami exponent in the 4.5–5.3 range [19]. PDO has a limited use, for example, as blow processed films due to its low crystallization rate and melt strength, together with its high cost and relatively low thermal stability. Therefore, non-isothermal crystallization studies have mainly focused on blends of PDO with other polymers such as poly(vinyl alcohol) [23] and polylactide [24]. It was assumed that the crystallization rate should be enhanced in blends. Thus, an improvement of properties and easier processing were determined. Polymers 2016, 8, 351 3 of 18

The purpose of the present work was to perform a complete analysis of non-isothermal crystallization considering both the overall kinetic process from DSC data and crystal growth rates from optical microscopy data. Furthermore, morphological changes occurring during heating and cooling processes were analyzed by real time synchrotron experiments. Dynamic diffraction data from samples degraded under different pHs were useful in confirming a lamellar organization inside fibers. In this way, recent studies on degraded monofilament sutures constituted by polyglycolide hard blocks highlighted a structural organization with interlamellar and interfibrillar domains [25].

2. Experimental Section Materials: Granulated PDO and processed PDO sutures (Monoplus™, USP 0) were kindly supplied by B. Braun Surgical, S.A (Rubí, Spain). In order to get PDO sutures, granulated polymer was gradually melted along the extruder barrel under an inert atmosphere applying a temperature gradient from 140 to 190 ◦C. The poly(p-dioxanone) polymer melt was metered by a spin pump with a specific throughput and extruded through a spinneret, which shapes the material to a monofilament fiber with a circular cross section. After passing through the spinneret, the fiber was quenched in a cold water bath to stabilize it. In order to obtain good tensile properties, the fiber was drawn and relaxed at certain drawing ratios, passing it through convection ovens. Finally, the fiber was taken up on drums winders. Number and weight average molecular weights determined by GPC were 112,800 and 250,000 for the granulated material and 103,000 and 226,300 for the commercial threads, respectively. Measurements: Molecular weights were estimated by size exclusion chromatography (GPC) using a liquid chromatograph (Shimadzu, model LC-8A, Kyoto, Japan) equipped with an Empower computer program (Waters, Mildford, MA, USA). A PL HFIP gel column (Polymer Lab) and a refractive index detector (Shimadzu RID-10A, Kyoto, Japan) were employed. The polymer was dissolved and eluted in 1,1,1,3,3,3-hexafluoroisopropanol containing CF3COONa (0.05 M) at a flow rate of 1 mL/min (injected volume 100 µL, sample concentration 2.0 mg/mL). The number and weight average molecular weights were calculated using polymethyl methacrylate standards. Calorimetric data were obtained by differential scanning calorimetry with a TA Instruments Q100 series (TA instruments, New Castle, DE, USA) with Tzero technology and equipped with a refrigerated cooling system (RCS, TA instruments, New Castle, DE, USA). Experiments were conducted under a flow of dry nitrogen with a sample weight of approximately 5 mg and calibration was performed with indium. Tzero calibration required two experiments: the first was performed without samples while sapphire disks were used in the second. Thermal characterization was conducted following a protocol consisting of a heating run (3 ◦C/min), a cooling run (3 ◦C/min) after keeping the sample in the melt state for 5 min to wipe out the thermal history, and a subsequent heating run (3 ◦C/min). Non-isothermal crystallization studies were performed by cooling the previously molten samples (5 min at 125 ◦C) at rates varying from 30 to 1 ◦C/min. The spherulitic growth rate was determined by optical microscopy using a Zeiss Axioskop 40 Pol light (Zeiss, Oberkochen, Germany) polarizing microscope equipped with a Linkam temperature control system (Tadworth, UK) configured by a THMS 600 heating and freezing stage connected to a LNP 94 liquid nitrogen cooling system (Tadworth, UK). Spherulites were grown from homogeneous thin films prepared from the melt. Small sections of these films were pressed or smeared between two cover slides and inserted into the hot stage, giving rise to samples with thicknesses close to 10 µm in all cases. Samples were kept at approximately 125 ◦C for 5 min to eliminate sample history effects. The radius of growing spherulites was monitored during crystallization with micrographs taken with a Zeiss AxiosCam MRC5 digital camera (Zeiss, Oberkochen, Germany) at appropriate time intervals. A first-order red tint plate was employed to determine the sign of spherulite birefringence under crossed polarizers. In vitro hydrolytic degradation assays were carried out at a physiological temperature of ◦ 37 C using a pH 7.4 phosphate buffer (Sörensen medium: 19.268 g of Na2HPO4·12H2O and 1.796 g of KH2PO4 in 1 L of deionized water) and a pH 11 from the Universal buffer Polymers 2016, 8, 351 4 of 18

(citrate-phosphate-borate/HCl) solution, mixing 20 mL of a stock solution with 14.7 mL of 0.1 M HCl and distilled water up to a volume of 100 mL. The stock solution (1 L) contained 100 mL of citric acid and 100 mL of phosphoric acid solution, each of which was equivalent to 100 mL NaOH 1 M, 3.54 g of boric acid, and 343 mL of 1 M NaOH. Samples were kept under orbital shaking in bottles filled with 50 mL of the degradation medium and sodium azide (0.03 wt %) to prevent microbial growth for selected exposure times. The samples were then thoroughly rinsed with distilled water, dried to constant weight under vacuum and stored over P4O10 before analysis. Finally, weight retention and molecular weight were evaluated. Time resolved SAXS (small-angle X-ray scattering) experiments were conducted at the NCD beamline (BL11) of the Alba synchrotron radiation light facility (Cerdanyola del Vallès, Spain). The beam was monochromatized to a wavelength of 0.1 nm. Polymer samples were confined between Kapton films and then held on a Linkam THMS600 hot stage (Tadworth, UK) with temperature control within ±0.1 ◦C. SAXS profiles were acquired during heating and cooling runs in time frames of 20 s and rates of 10 ◦C/min. The SAXS patterns were calibrated with diffractions of a standard of a silver behenate sample. The diffraction profiles were normalized to the beam intensity and corrected considering the empty sample background. The correlation function and corresponding parameters were calculated with the CORFUNC program for Fiber Diffraction/Non-Crystalline Diffraction provided by Collaborative Computational Project 13 (CCP13) (Chester, UK).

3. Results and Discussion

3.1. Melting and Crystallization of Poly(p-dioxanone) Thermal behavior of PDO is rather complicated, as revealed by the DSC curves in Figure1. Heating traces are clearly different for commercial granulated samples crystallized from the melt and processed surgical sutures, demonstrating the significant influence of thermal treatments on melting behavior. Thus, two well-differentiated endothermic melting peaks at 97 ◦C (small) and 107 ◦C were observed when samples crystallized from the melt state at a rate of 3 ◦C/min. These peaks have been largely discussed [6,7] and attributed to a typical lamellar reorganization process that leads to lamellar thickening during heating. Commercial granulated PDO showed that the high temperature peak was split into two equally intense peaks at 104 and 107 ◦C. Therefore, the population of thinner crystals was not present in the manufactured PDO form or at least they were sufficiently energetically unstable to lead to a complete recrystallization process on heating. Probably two populations of new crystals with practically the same thickness existed. The double peak appearing in the 104–107 ◦C range (see label at 105 ◦C) was also detected in the heating run of the as-processed suture, but in this case the population of thinner crystals was highly stable and did not undergo a recrystallization process. Note the intense and narrow peak at 98 ◦C, which in this case was observed as a consequence of the high temperature annealing process at which commercial sutures were submitted. Figure2 shows the dynamic DSC exotherms obtained by cooling the melted samples at different rates. Crystallization peaks progressively shift to lower temperatures as the cooling rate increases, as expected, but peaks become broader. Two different crystallization processes seemed to occur at cooling rates equal or higher than 15 ◦C/min, a feature that is later discussed on the basis of different nucleation mechanisms (i.e., instantaneous and sporadic at low and high temperatures, respectively).

3.2. Non-Isothermal Kinetic Analysis of Poly(p-dioxanone) Melt Crystallization from DSC Data The process of crystallization under non-isothermal conditions is rather complicated to be analyzed since crystallization from the melt takes place under different degrees of supercooling, and therefore caution should be taken when interpreting experimental results. Polymers 2016, 8, 351 5 of 18 Polymers 2016, 8, 351 5 of 18 Polymers 2016, 8, 351 5 of 18

Figure 1. Differential scanning calorimetry (DSC) traces corresponding to heating runs of commercial Figure 1. Differential scanning calorimetry (DSC) traces corresponding to heating runs of commercial granulatedFigure 1. Differential polydioxanone scanning (PDO) calorimetry (a) and PDO (DSC) sutures traces (correspondingd), the cooling torun heating of the runsmelted of commercialgranulated granulated polydioxanone (PDO) (a) and PDO sutures (d), the cooling run of the melted granulated PDOgranulated (b) and polydioxanone the subsequent (PDO) heating (a) andrun PDO(c). Glass sutures transition (d), the cancooling be detected run of the in meltedthe magnification granulated PDO (b) and the subsequent heating run (c). Glass transition can be detected in the magniﬁcation given givenPDO (inb) theand inset the insubsequent (c). All scans heating were runperforme (c). Glassd at atransition rate of 3 °C/min.can be detected in the magnification ◦ in thegiven inset in in the (c ).inset All in scans (c). All were scans performed were performe at a rated at a of rate 3 C/min.of 3 °C/min.

Figure 2. Exothermic DSC traces performed with PDO at the indicated cooling rates. The dashed ellipse contains the high temperature crystallization peak detected at high cooling rates, whereas the FigureFigure 2. Exothermic 2. Exothermic DSC DSC traces traces performed performed withwith PDO at at the the indicated indicated cooling cooling rates. rates. The Thedashed dashed dashedellipse contains arrow indicates the high the temperature evolution ofcrystallization the main crystallization peak detected peak. at high cooling rates, whereas the ellipse contains the high temperature crystallization peak detected at high cooling rates, whereas the dashed arrow indicates the evolution of the main crystallization peak. dashedCalorimetric arrow indicates data were the evolution used to determine of the main the crystallization relative degree peak. of crystallinity at any temperature, χ(T), Calorimetricfor all cooling data rates were by theused expression: to determine the relative degree of crystallinity at any temperature,

Calorimetricχ(T), for all cooling data wererates by used the toexpression: determine theT relative degree of crystallinity at any temperature, c () d/ddHTTc χ(T), for all cooling rates by the expression: TTc χ = 0 ()T T ()d/ddHTT (1) T ∞ c 0 χ ()T = (dHTTc / d )d TTc∞ (1) 0 (dH /dT) dT T0 (dHTTc / d )d χ(T) = rT c (1) T0∞ (dHc/dT)dT rT0 where dHc is the enthalpy of crystallization released within an inﬁnitesimal temperature range dT, T0 denotes the initial crystallization temperature and Tc and T∞ are the crystallization temperatures PolymersPolymers2016 2016, 8, 351, 8, 351 6 of 186 of 18

where dHc is the enthalpy of crystallization released within an infinitesimal temperature range dT, at timeT0 denotest and the after initial completion crystallization of the temperature crystallization and T process,c and T∞ are respectively. the crystallization Thus, temperatures the denominator at correspondstime t and to theafter overall completion enthalpy of the of crystallizationcrystallization process, for speciﬁc respectively. heating/cooling Thus, the conditions. denominator correspondsThe relative to degree the overall of crystallinity enthalpy of wascrystall calculatedization for as specific a function heating/cooling of time by theconditions. relationship The relative degree of crystallinity was calculated as a function of time by the relationship t − t T − T φ ( (t 0−) t0) = = ( (T00 − T)/φ )/ (2) (2)

where T0 is the temperature at which crystallization begins (t = t0) and φ is the value of the cooling where T0 is the temperature at which crystallization begins (t = t0) and φ is the value of the cooling rate. rate. Figure3a illustrates the variation of the time-dependent degree of crystallinity, χ(t − t0), at Figure 3a illustrates the variation of the time-dependent degree of crystallinity, χ(t − t 0 ), at different cooling rates, which allows a typical Avrami analysis to be performed [20] according to different cooling rates, which allows a typical Avrami analysis to be performed [20] according to the equation the equation n 1 − χ(t − t0) = exp[−Z(t − t0) ], (3) 1 − χ(t − t0) = exp[−Z(t − t0)n], (3) where Z is the rate constant and n is the Avrami exponent. A normalized rate constant, k = Z1/n, is where Z is the rate constant and n is the Avrami exponent. A normalized rate constant, k = Z1/n, is however more useful for comparison because its dimension (time−1) becomes independent of the however more useful for comparison because its dimension (time−1) becomes independent of the Avrami exponent. Avrami exponent. Figure3b shows the plots of log{ −ln[1 − χ(t − t )]} versus log(t − t ) at different cooling Figure 3b shows the plots of log{−ln[1 − χ(t − t0)]} versus0 log(t − t0) at different0 cooling rates. A rates.good A goodlinearity linearity was observed was observed between between the relative the degr relativeee of degree crystallinities of crystallinities of 0.10 and of 0.90, 0.10 that and is, 0.90, thatafter is, after formation formation of well-defined of well-deﬁned spherulitic spherulitic morp morphologieshologies and andbefore before occurrence occurrence of a ofsecondary a secondary crystallizationcrystallization caused caused by by the the impingement impingement of of spherulitesspherulites (see (see dashed dashed lines lines in inFigure Figure 3a).3a).

Figure 3. (a) Time evolution of relative crystallinity at the indicated cooling rates for non-isothermal Figure 3. (a) Time evolution of relative crystallinity at the indicated cooling rates for non-isothermal crystallization of PDO; (b) Avrami analyses of non-isothermal crystallizations of PDO. crystallization of PDO; (b) Avrami analyses of non-isothermal crystallizations of PDO. Table 1 summarizes the main kinetic parameters calculated by the Avrami analysis. As known fromTable isothermal1 summarizes studies, the the main normalized kinetic parameters rate constant calculated was low by(i.e., the between Avrami 0.58 analysis. × 10−3 Ass−1 knownand − − from15.26 isothermal × 10−3 s−1) studies, and increased the normalized with the cooling rate constantrate. Avrami was exponents low (i.e., showed between a moderate 0.58 × 10 variation3 s 1 and 15.26(i.e.,× 10between−3 s−1 )3.76 and and increased 2.77), with with the the lowest cooling valu rate.es being Avrami determined exponents for showedhigh cooling a moderate rates and variation the (i.e.,average between value 3.76 being and close 2.77), to with3.0. These the lowestexponents values are lower being than determined those previously for high reported cooling by Zhang rates and the averageet al. [24] value (i.e., 4.26–3.40) being close and to in 3.0. good These agreemen exponentst with are those lower given than by those Andjelic previously et al. [4] reportedfor low by Zhangcrystallization et al. [24] (i.e., rates 4.26–3.40) (i.e., 3.0), although and in good in this agreement case a value with of those 1.1 was given found by for Andjelic high crystallization et al. [4] for low crystallizationrates. Isothermal rates (i.e.,crystallization 3.0), although studies in this also case indi acate value slightly of 1.1 contradictory was found for values. high crystallizationThus, minimum rates. changes with crystallization temperature were determined by Andjelic et al. (i.e., exponents varied Isothermal crystallization studies also indicate slightly contradictory values. Thus, minimum changes between 2.22 and 2.62, with 2.5 being the average value) [4], but a systematic increase (i.e., from with crystallization temperature were determined by Andjelic et al. (i.e., exponents varied between approximately 2 to 3.8) was also reported [6] for higher isothermal crystallization temperatures (i.e., 2.22 and 2.62, with 2.5 being the average value) [4], but a systematic increase (i.e., from approximately 2 to 3.8) was also reported [6] for higher isothermal crystallization temperatures (i.e., from 30 to 80 ◦C). The last behavior was interpreted as a consequence of a change from instantaneous to sporadic nucleation as Tc was increased [6]. Polymers 2016, 8, 351 7 of 18

It can be well stated that application of the Avrami equation under non-isothermal conditions merely corresponds to a mathematical ﬁtting that allows appropriate values of the rate constant to be derived [26–28]. In this case, it should be pointed out that the determined exponents may even have a physical meaning since they suggest three-dimensional spherulitic growth and instantaneous nucleation, as postulated from isothermal studies [4,6]. Furthermore, the sporadic nucleation detected at high isothermal crystallization temperatures [6] is in agreement with the increase of the exponent observed at low crystallization rates (i.e., 3.76 at 1 ◦C/min) and supports DSC evidence of the occurrence of two crystallization processes.

Table 1. Main non-isothermal crystallization kinetic parameters of polydioxanone (PDO) determined by differential scanning calorimetry (DSC).

◦ −n 3 −1 3 −1 1/n 3 −1 φ ( C/min) n Z (s ) k × 10 (s ) τ1/2 (s) (1/τ1/2) × 10 (s ) (Z/ln2) × 10 (s ) 1 3.76 6.79 × 10−13 0.58 1,574 0.64 0.64 3 3.11 7.90 × 10−10 1.18 749 1.34 1.32 5 3.30 1.37 × 10−9 2.06 428 2.34 2.31 8 3.10 1.83 × 10−8 3.21 272 3.68 3.61 10 2.92 1.45 × 10−7 4.55 192 5.21 5.16 15 2.97 2.76 × 10−7 6.17 142 7.07 6.98 20 2.73 2.25 × 10−6 8.60 104 9.65 9.83 30 2.77 9.12 × 10−6 15.26 58 17.14 17.42

The values of the corresponding reciprocal crystallization half times (1/τ1/2), calculated as the inverse of the difference between crystallization starting time and crystallization half time, are also given in Table1. This parameter is a direct measure of the crystallization process, and could therefore be used to check the accuracy of Avrami analyses, as demonstrated by the excellent agreement with 1/n the theoretical kinetic value (i.e., 1/τ1/2 = (Z/ln2) ). In conclusion, the deduced Avrami parameters are completely appropriate to simulate the non-isothermal crystallization process. A kinetic equation that combines the Avrami [20] and Ozawa [17] expressions has been derived and applied in different non-isothermal studies [29]:

log φ = log F(T) − a log(t − t0) (4) where F(T) is a new kinetic parameter referring to the cooling rate which must be chosen at a unit crystallization time when the system reaches a certain crystallinity, and a is the ratio of apparent Avrami and Ozawa exponents. A plot of log φ versus log (t − t0) yields a series of straight lines at a given value of χ(T) (Figure4), which suggest the validity of the combined equation for this system. Kinetic parameters can be estimated by the intercept and slope of these lines. Results showed that F(T) values increased with crystallinity (Table2), which has a physical sense because the motion of the molecular chains became slower as the material crystallized and formation of new crystals was hindered. The values of a were almost constant between 0.91 and 0.98 although slightly increased with the relative degree of crystallinity. The crystallization process has non-Arrhenius behavior, and therefore a temperature-dependent effective activation energy needs to be deﬁned. The value corresponding to a given degree of crystallinity, Eχ, can be determined by the Friedman isoconversional method [30]:

[dχ/dt]χ = Aexp(−Eχ/RT)f [χ], (5) where A is a pre-exponential factor and f [χ] is the crystallization model. Values of ln[dχ/dt]χ at different temperatures and degrees of crystallization can be obtained from crystallization experiments performed at different cooling rates. In this way, the slopes of the linear plots of ln[dχ/dt]χ versus 1/T (Figure5a) allow Eχ to be determined (Figure5b). The temperature dependence of the effective Polymers 2016, 8, 351 8 of 18

Polymers 2016, 8, 351 8 of 18

Polymers 2016, 8, 351 8 of 18 activationof the energyeffective (Figureactivation5c) energy could (Figure ﬁnally 5c) be could derived finally by be considering derived by considering also the average also the temperature average associatedtemperature with a associated given conversion with a given (Figure conversion5b). (Figure 5b). of the effective activation energy (Figure 5c) could finally be derived by considering also the average temperature associated with a given conversion (Figure 5b).

φ Figure 4. Plots of log versus log (t − t0) for non-isothermal crystallization of PDO performed at the Figure 4. Plots of log (φ) versus log (t − t0) for non-isothermal crystallization of PDO performed at the indicated crystallinities. indicatedFigure crystallinities. 4. Plots of logφ versus log (t − t0) for non-isothermal crystallization of PDO performed at the indicated crystallinities.

Figure 5. (a) Plots of ln[dχ/dt]χ versus 1/T for non-isothermal crystallization of PDO at the indicated cooling rates. Data corresponding to relative degrees of crystallinity of 0.8, 0.5 and 0.1 are represented Figure 5. (a) Plots of ln[dχ/dt]χ versus 1/T for non-isothermal crystallization of PDO at the indicated Figureby5. blue,(a) Plots green of ln[dand χred/dt] symbols,χ versus 1/respectively;T for non-isothermal (b) Dependence crystallization of the activation of PDO atenergy the indicated of cooling rates. Data corresponding to relative degrees of crystallinity of 0.8, 0.5 and 0.1 are represented coolingcrystallization rates. Data (● corresponding) and average temperature to relative degrees(○) on crystallinity; of crystallinity (c) Experimental of 0.8, 0.5 andEχ versus 0.1 are T plot represented and by blue, green and red symbols, respectively; (b) Dependence of the activation energy of by blue,simulated green andcurves red according symbols, to respectively; Equation (6). Red (b) Dependenceand violet dashed of the lines activation indicate energythe simulated of crystallization curves crystallization (●) and average temperature (○) on crystallinity; (c) Experimental Eχ versus T plot and for regimes III and II, respectively. Arrows indicate the expected temperatures for the maximum ( ) andsimulated average curves temperature according (to Equation) on crystallinity; (6). Red and (c violet) Experimental dashed linesE indicateχ versus theT simulatedplot and curves simulated crystallization rates (i.e., effective activation energy equal to zero). curvesfor according regimes III to and Equation II, respectively. (6).# Red and Arrows violet indi dashedcate the lines expected indicate temperatures the simulated for curvesthe maximum for regimes III andcrystallization II, respectively. rates (i.e., Arrows effective indicate activation the expected energy equal temperatures to zero). for the maximum crystallization The effective activation energy was negative for crystallization experiments performed from the rates (i.e., effective activation energy equal to zero). melt state and at low supercooling degrees (i.e., the temperature range where secondary nucleation The effective activation energy was negative for crystallization experiments performed from the plays a fundamental role), as shown in Figure 5c. This energy increased progressively (i.e., the melt state and at low supercooling degrees (i.e., the temperature range where secondary nucleation The effective activation energy was negative for crystallization experiments performed from plays a fundamental role), as shown in Figure 5c. This energy increased progressively (i.e., the the melt state and at low supercooling degrees (i.e., the temperature range where secondary nucleation plays a fundamental role), as shown in Figure5c. This energy increased progressively Polymers 2016, 8, 351 9 of 18

(i.e., the crystallization rate increased) as the temperature decreased, as discussed at length by Vyazovkin and Dranca [31], reﬂecting the expected behavior for crystallizations performed at temperatures higher than those associated with the maximum crystallization rate. Vyazovkin and Sbirrazzuoli proposed that crystallization parameters like the secondary nucleation constant should be derived through the effective activation energies [31–33]:

−1 2 2 ◦ 2 E(T) = −RdlnG/dT = U*[T /(T − T∞) ] + KgR[(2∆T − Tm f )/(∆T) f ] (6) where G is the crystal growth rate, U* represents the activation energy characteristic of the transport of crystallizing segments across the liquid-crystal interface, T∞ is the temperature below which such motion ceases, R is the gas constant, Kg is the secondary nucleation constant, ∆T is the degree of supercooling measured as Tm − Tc (where Tm is the equilibrium melting temperature and Tc is the crystallization temperature), and f is a correction factor accounting for the variation in the bulk melting enthalpy per unit volume with temperature (f = 2Tc/(Tm + Tc)). Figure5c also compares the experimental Eχ − T plot with simulated ones using Equation (6), U* and T∞ values of 1600 cal/mol and Tg-35 K, respectively (i.e., close to the universal values reported by Suzuki and Kovacs [34]), the equilibrium melting temperature of 127 ◦C, as previously determined for PDO [3], and representative Kg values. In fact, U* and T∞ have little inﬂuence on a temperature range that is far from the glass transition temperature. The best ﬁt between experimental and theoretical 5 2 5 2 data was obtained considering two Kg parameters (i.e., 3.07 × 10 K and 1.42 × 10 K ), which are in agreement with the two crystallization regimes reported for PDO from isothermal crystallization experiments [3]. Note that the isoconversional analysis was able to detect the existence of several crystallization regimes and also to predict two maximum crystallization rates at temperatures of 45 and 60 ◦C, as deduced from the temperatures for each simulated curve where the effective activation energy was zero.

Table 2. Values of kinetic parameters at a given crystallinity estimated from the combined model [35] for non-isothermal crystallization of PDO.

χ(T) a F(T) r2 0.1 0.91 12.67 0.993 0.2 0.93 16.98 0.995 0.3 0.94 20.79 0.996 0.4 0.95 24.27 0.996 0.5 0.96 27.64 0.996 0.6 0.96 31.12 0.996 0.7 0.97 35.04 0.995 0.8 0.98 41.51 0.995 0.9 0.98 48.11 0.993

3.3. Non-Isothermal Kinetic Analysis of Poly(p-dioxanone) Melt Crystallization from Optical Microscopy Data Non-isothermal crystallization of PDO rendered double banded spherulites with progressively decreasing periodicity (Figure6), which is in accordance with the continuous temperature decrease of a non-isothermal crystallization. In fact, morphologies obtained under isothermal conditions have been extensively studied [3,5,6], and it was assumed that interband spacing decreased when crystallization temperature was lowered. Actually, two different banding periodicities could be detected where the broader bands had a negative birefringence. These kinds of double bands with uneven spacings are characteristic of spherulites having a biaxial indicatrix twisted about the optic normal [6,32,35]. PDO spherulites were also characterized by their big size, which led to poor nucleation and slow growth rate. Figure6 also shows that the number of active nuclei increased during cooling, and consequently the size and morphology of the spherulites were not identical. Logically smaller spherulites with a practically indistinguishable double band texture were formed at lower temperatures (see yellow arrows). Polymers 2016, 8, 351 10 of 18 Polymers 2016, 8, 351 10 of 18

Figure 6. OpticalOptical micrograph micrograph of PDO PDO spherulites spherulites formed during a non-isothermal non-isothermal crystallization from the melt state performed atat aa coolingcooling raterate ofof 2020 ◦°CC/min./min. Yellow Yellow arrows arrows point to spherulites formed at low temperatures. Inset Inset shows shows a a micrograph taken taken wi withth a first-order ﬁrst-order red tint plate to determine the birefringence sign.

Spherulitic growth rates (G) were also determined for non-isothermal crystallization by Spherulitic growth rates (G) were also determined for non-isothermal crystallization by measuring measuring the change of the spherulite radius (R) with temperature (T) at a constant cooling/heating the change of the spherulite radius (R) with temperature (T) at a constant cooling/heating rate rate (dT/dt) [36,37]: (dT/dt)[36,37]: G =G d =R d/dR/dt t= = (d R/dTT) )× (d×T(d/dTt)/d t)(7) (7)

Plots showing the variation of the spherulitic radiusradius with crystallization temperature could be adjusted toto thirdthird orderorder equations equations with with good good regression regression coefficients coefficients (i.e., (i.e., higher higher than than 0.990) 0.990) (Figure (Figure7a). These7a). These coefficients coefficients were were significantly significantly better better than than those those calculated calculated for for second second orderorder equationsequations and remained constant for higher orders. Therefore, th thirdird order equations were employed to determine dR/dTT asas a a function function of of the the crystallization crystallization temperature. temperature. The The corresponding corresponding crystal growth rate versus crystallization temperature curves are displayed in FigureFigure7 b.7b. NoteNote thatthat datadata werewere obtainedobtained atat different coolingcooling ratesrates inin orderorder to to maximize maximize the the crystallization crystallization temperature temperature range range where where radii radii could could be wellbe well measured. measured. Two bell-shaped curves with maximums of 45 and 60 ◦°CC reflectedreflected the temperature dependence of G, andand thereforetherefore thethe existenceexistence ofof twotwo crystallizationcrystallization regimesregimes withwith differentdifferent secondarysecondary nucleationnucleation constants. These were determined by the Lauritzen-Hoffman equation [[38]:38]:

G = G0exp[−U*/(R(Tc − T∞))] × exp[−Kg/(Tc(ΔT)f)] (8) G = G0exp[−U*/(R(Tc − T∞))] × exp[−Kg/(Tc(∆T)f )] (8) where G0 is the constant pre-exponential factor and the other parameters as previously defined. whereFigureG0 is the7c shows constant the pre-exponential linear plots obtained factor and using the otherU* and parameters T∞ parameters as previously of 1600 defined.cal/mol and Tg-35Figure K, respectively.7c showsthe It is linear clear plotsthat two obtained crysta usingllizationU* regimesand T∞ definedparameters by secondary of 1600 cal/mol nucleation and Tconstantsg-35 K, respectively. of 3.07 × 105 K It2 isand clear 1.42 that × 10 two5 K2 crystallizationfit the experimental regimes data. defined Furthermore, by secondary regimes nucleation III and II constantscould be assumed of 3.07 × since105 K 2theand experimental 1.42 × 105 K 2ratiofit the betw experimentaleen slopes data.(2.16) Furthermore,was close to regimesthe theoretical III and IIKg couldIII/KgII value be assumed of 2. since the experimental ratio between slopes (2.16) was close to the theoretical III II Kg /FigureKg value 7b also of 2.shows that the two bell-shaped curves calculated by equation 8, the estimated U* and FigureT∞ parameters,7b also shows and thatthe deduced the two bell-shaped values of ln curves G0 and calculated Kg for each by Equation regime (8),fit thewell estimated with the U*experimentaland T∞ parameters, spherulitic andgrowth the data. deduced The valuesmaximum of ln growthG0 and rateKg forwas each found regime in regime fit well III withand thecorresponded experimental to a spherulitic temperature growth of 45 data. °C. TheOur maximumobservations growth are in rate agreement was found with in regimethe same III andcrystallization corresponded regimes to a determined temperature from of 45isothermal◦C. Our crystallization observations arealthough in agreement the nucleation with the constant same crystallizationbecomes slightly regimes higher determined than those previously from isothermal reported crystallization (i.e., 2.49 × 10 although5 K2 and the 1.19 nucleation × 105 K2) [3]. constant Note becomesalso that slightlythe deduced higher data than support those previously the results reported of the calorimetric (i.e., 2.49 × study.105 K2 and 1.19 × 105 K2)[3]. Note also that the deduced data support the results of the calorimetric study.

Polymers 2016, 8, 351 11 of 18 Polymers 2016, 8, 351 11 of 18

Polymers 2016, 8, 351 11 of 18

Figure 7. (a) Variation in spherulite radius with temperature during heating at the indicated rates; Figure 7. (a) Variation in spherulite radius with temperature during heating at the indicated rates; (b) Spherulitic growth rates determined by the equations deduced for the heating runs. Theoretical (b) Spherulitic growth rates determined by the equations deduced for the heating runs. Theoretical curves are also drawn (dashed lines) for comparative purposes; (c) Plot of lnG + U*/R(Tc − T∞) versus curves are also drawn (dashed lines) for comparative purposes; (c) Plot of lnG + U*/R(Tc − T∞) versus 1/Tc(ΔT)f to determine the Kg secondary nucleation parameters of PDO. 1/Tc(Figure∆T)f to 7.determine (a) Variation the inK spheruliteg secondary radius nucleation with temperat parametersure during of PDO. heating at the indicated rates; The(b) Spherulitic fact that thegrowth crystallization rates determined rate is by govern the equationsed by two deduced different for the processes heating runs. makes Theoretical it unfeasible Theto determine factcurves that are a the alsosinglecrystallization drawn activation (dashed energy lin ratees) for isfor governedcomparative the entire by Tpurposes;c range. two different Instead,(c) Plot of an processes ln Geffective + U*/R( makes Tactivationc − T∞) itversus unfeasible energy to (E) dependent1/Tc(ΔT)f to on determine Tc was evaluatedthe Kg secondary by equation nucleation 9 [32]: parameters of PDO. determine a single activation energy for the entire Tc range. Instead, an effective activation energy (E) −1 2 2 2 2 2 dependentThe on factTc wasthatE the evaluated= − RcrystallizationdlnG/d byT Equation= U rate*T /( isT govern− (9) T∞) [32 +ed ]:Kg Rby[( Ttwom°) different − T −TmT processes]/[(Tm − T) makesT] it unfeasible(9) to determine a single activation energy for the entire Tc range. Instead, an effective activation energy The calculated− effective1 activation2 energies2 are plotted ◦in2 Figure 28 and show non-Arrhenius2 E = −RdlnG/dT = U*T /(T − T∞) + KgR[(Tm ) − T −TmT]/[(Tm − T) T] (9) behavior(E) dependent as expected. on Tc was Different evaluated Kg byvalues equation were 9 used[32]: according to the crystallization regime. The

effective activationE =energy −Rdln Gis/d zeroT−1 = at U the*T2/( maximumT − T∞)2 + KcrystallizationgR[(Tm°)2 − T2− rateTmT ]/[(forT regimesm − T)2T ]III and II, which(9) Thecorresponds calculated to temperatures effective activation of 45 and 60 energies °C and agree are plotted again with in Figurethose deduced8 and showby isoconversional non-Arrhenius behavioranalysis.The as expected.calculatedIn each case, effective Different positive activation valuesKg values areenergies found were ar fore usedplotted temperatures according in Figure lower to8 and thethan show crystallizationthe non-Arrheniuscorresponding regime. The effectivemaximabehavior activationbecauseas expected. the energy crystallization Different is zero Kg atvaluesrate the increasesmaximum were used with crystallizationaccording increasing to temperature,the rate crystallization for regimes whereas regime. III andnegative II,The which correspondsvalueseffective are toactivation determined temperatures energy for higher ofis 45zero andtemperatures at the 60 ◦maximumC and charac agree crystallizationterized again by with a decrease rate those for deduced regimesof the crystallization III by and isoconversional II, which rate analysis.withcorresponds increasing In each to case, temperaturestemperature. positive of values 45 and 60 are °C found and agree for again temperatures with those lower deduced than by isoconversional the corresponding analysis. In each case, positive values are found for temperatures lower than the corresponding maxima because the crystallization rate increases with increasing temperature, whereas negative maxima because the crystallization rate increases with increasing temperature, whereas negative values are determined for higher temperatures characterized by a decrease of the crystallization rate values are determined for higher temperatures characterized by a decrease of the crystallization rate with increasingwith increasing temperature. temperature.

Figure 8. Dependence of effective activation energy on crystallization temperature for regimes II (■) and III (•). Extrapolated data for regimes II and III are indicated by dotted and dashed lines, respectively.

Figure 8. Dependence of effective activation energy on crystallization temperature for regimes II (■) Figure 8. Dependence of effective activation energy on crystallization temperature for regimes II () and and III (•). Extrapolated data for regimes II and III are indicated by dotted and dashed lines, III (•). Extrapolated data for regimes II and III are indicated by dotted and dashed lines, respectively. respectively.

Polymers 2016, 8, 351 12 of 18 Polymers 2016, 8, 351 12 of 18

3.4. Evolution of Morphologic Parameters during HeatingHeating ofof Poly(p-dioxanone)Poly(p-dioxanone) SamplesSamples Figure9 9shows shows the the evolution evolution of theof intensitythe intensity of the of peak the detectedpeak detected in SAXS in patterns SAXS patterns during heating during andheating cooling and processescooling processes of the granulated of the granulated PDO sample. PDO In sample. the ﬁrst case,In the a recrystallizationfirst case, a recrystallization process that ledprocess to thicker that led lamellae to thicker can belamellae deduced can from be deduced the increase from in the SAXS increase peak in intensity SAXS peak and itsintensity shift towards and its lowershift towards values oflower the scatteringvalues of the vector scattering (q = [4 vectorπ/λ] sen (q θ=). [4 Inπ/λ the] sen secondθ). In case,the second a shift case, of the a shift peak of once the samplespeak once were samples crystallized were crystallized towards higher towardsq values higher and q a values slightdecrease and a slight on its decrease intensity on was its observed.intensity Thesewas observed. features These can be features explained can considering be explained that cons aidering lamellar that insertion a lamellar mechanism insertion took mechanism place at took low temperaturesplace at low togethertemperatures with atogether densiﬁcation with ofa thedensi amorphousfication of phase the amorphous (i.e., a smaller phase difference (i.e., a between smaller thedifference density between of amorphous the density and crystalline of amorphous phases). and crystalline phases).

2 Figure 9. VariationVariation of of intensity intensity (Iq (Iq) (full2) (full symbols) symbols) and and scattering scattering vector vector (q) (empty (q) (empty symbols) symbols) of SAXS of SAXS(small-angle (small-angle X-ray X-rayscattering) scattering) peaks peaks observed observed in the in the diffraction diffraction profiles proﬁles taken taken during during heatingheating (10(10 ◦°C/min)C/min) at at room room temperature temperature (red) (red) and and during cooling (2 °C/min)◦C/min) from from the the melt melt state state (blue). (blue).

Characteristic lamellar parameters (i.e., long period, Lγ, amorphous layer thickness, la, and Characteristic lamellar parameters (i.e., long period, Lγ, amorphous layer thickness, la, and crystalline lamellar thickness, lc) and crystallinity (i.e., crystallinity within the lamellar stacks, crystalline lamellar thickness, lc) and crystallinity (i.e., crystallinity within the lamellar stacks, SAXS XcSAXS = lc/Lγ) were determined by the normalized one-dimensional correlation function [35], γ(r): Xc = lc/Lγ) were determined by the normalized one-dimensional correlation function [35], γ(r): ∞ ∞ 2 2 γ(r) = qIq∞ ()cos()d qrq/∞ qIq()d q (10) 0 2 0 2 γ(r) = w q I(q)cos(qr)dq/w q I(q)dq (10) where I(q) is the intensity at each value0 of the scattering vector.0 whereSAXSI(q) isdata the were intensity collected at each within value a oflimited the scattering angular vector.range, with application of the Vonk’s model [39] andSAXS Porod’s data were law collectedto perform within extrapolations a limited angular to low range, and high with q applicationvalues. of the Vonk’s model [39] and Porod’sFigure 10 law illustrates to perform representative extrapolations correlation to low and functions high q values.obtained from patterns acquired during the heatingFigure 10of illustratesgranulated representative PDO. Lamellar correlation thickening functions was due obtained to the increase from patterns in crystalline acquired lamellar during thethickness heating (i.e., of granulatedfrom 6.0 to PDO.7.7 nm) Lamellar and amorphous thickening la wasyer thickness due to the (i.e., increase from in1.5 crystalline to 1.8 nm). lamellar In this thicknessway, the reordering (i.e., from process 6.0 to 7.7 led nm) to a and minimum amorphous increase layer of thicknessthe crystallinity (i.e., from within 1.5 lamellar to 1.8 nm). stacks In (i.e., this way,from the0.80 reordering to 0.81). However, process led it to should a minimum be point increaseed out of that the crystallinitythe correlation within function lamellar of the stacks sample (i.e., fromheated 0.80 up to to 0.81 102). °C However, (i.e., just it before should the be first pointed melting out thatpeak the observed correlation in the function DSC trace) of the exhibited sample heated more updefined to 102 peaks◦C (i.e., (see just also before the diffraction the first melting patterns peak in observedFigure 10) in that the DSCwere trace) indicative exhibited of a morehigh contrast defined peaksbetween (see electronic also the diffractiondensities of patterns amorphous in Figure and crystalline10) that were phases. indicative Basically, of a highthe amorphous contrast between phase electronicbecame less densities dense ofin amorphous agreement andwith crystalline the maximum phases. value Basically, detected the amorphous for la (i.e., phase 1.9 nm) became and lessthe minimum value of crystallinity (i.e., 0.78). Figure 10 also shows that morphological features were dense in agreement with the maximum value detected for la (i.e., 1.9 nm) and the minimum value of crystallinitycompletely recovered (i.e., 0.78). after Figure cooling 10 also the shows sample that and morphological specifically the features correlation were completely functions and recovered the X- afterray diffraction cooling the pattern sample were and identical specifically to those the correlation obtained from functions the initial and sample. the X-ray diffraction pattern were identical to those obtained from the initial sample.

Polymers 2016,, 8,, 351351 13 of 18

Figure 10. ((aa)) SAXS SAXS patterns patterns of of a agranulated granulated PDO PDO sample sample taken taken at 25 at 25and and 102 102 °C during◦C during a heating a heating run runperformed performed at 10 at °C/min; 10 ◦C/min; (b) Change (b) Change in the in correlation the correlation function function during during the heating the heating run. For run. the For sake the sakeof completeness of completeness the pattern the pattern and correlation and correlation function function obtained obtained at room at room temperature temperature after after cooling cooling (10 (10°C/min)◦C/min) a previously a previously molten molten sample sample are also are alsoshown. shown.

The evolution of SAXS patterns of a PDO thread on heating (Figure 11) was very different The evolution of SAXS patterns of a PDO thread on heating (Figure 11) was very different because because they reflect the microstructure of the processed sample. they reﬂect the microstructure of the processed sample.

(a) (b)

Figure 11. (a) SAXS patterns of a PDO suture taken at representative temperatures during a heating runFigure at 1011.◦ C/min.(a) SAXS Blue patterns and redof aarrows PDO suture indicate taken meridional at representative and equatorial temperatures reflections, during respectively; a heating (runb) Correlationat 10 °C/min. function Blue and of red diffraction arrows indicate patterns meri correspondingdional and equatorial to: the initial reflections, suture, respectively; a suture heated (b) (10Correlation◦C/min) function just before of diffraction melting andpatterns a melt correspon crystallizedding to: (cooling the initial rate sutu ofre, 10 a◦ C/min)suture heated suture (10 at room°C/min) temperature. just before melting and a melt crystallized (cooling rate of 10 °C/min) suture at room temperature. Some observations can be made: (a) The initial pattern was characterized by a meridional reflection Some observations can be made: (a) The initial pattern was characterized by a meridional that indicates a stacking of lamellae perpendicularly oriented to the fiber axis. Peaks observed in the reflection that indicates a stacking of lamellae perpendicularly oriented to the fiber axis. Peaks corresponding correlation function were highly prominent suggesting a tie molecular arrangement observed in the corresponding correlation function were highly prominent suggesting a tie molecular in the dense crystalline phase. However, lc and la values (6.1 nm and 1.5 nm) were close to those arrangement in the dense crystalline phase. However, lc and la values (6.1 nm and 1.5 nm) were close determined for the granulated sample; (b) As the temperature was increased the interlamellar reflection to those determined for the granulated sample; (b) As the temperature was increased the decreased in intensity while a new perpendicular reflection appeared and progressively increased interlamellar reflection decreased in intensity while a new perpendicular reflection appeared and in intensity. This new equatorial reflection could be associated with the existence of interfibrillar progressively increased in intensity. This new equatorial reflection could be associated with the amorphous domains that correspond to the regions placed on the lateral sides of lamellae. Note that existence of interfibrillar amorphous domains that correspond to the regions placed on the lateral molecular chains in these domains may have had a partial orientation at the beginning of the heating sides of lamellae. Note that molecular chains in these domains may have had a partial orientation at (i.e., the as processed thread) but became more randomly distributed as the temperature was increased. the beginning of the heating (i.e., the as processed thread) but became more randomly distributed as Therefore, a decrease in electronic density of the amorphous phase and an enhancement of the intensity the temperature was increased. Therefore, a decrease in electronic density of the amorphous phase and an enhancement of the intensity of the SAXS reflection were derived. The correlation function of

Polymers 2016, 8, 351 14 of 18 Polymers 2016, 8, 351 14 of 18 ofthe the interfibrillar SAXS reflection reflection were observed derived. in The the correlation pattern taken function just before of the interfibrillarfusion gave l reflectionc and la values observed (i.e., in9.7 the nm pattern and 2.2 taken nm, just respectively), before fusion which gave werelc and clearlyla values different (i.e., 9.7 from nm andthose 2.2 determined nm, respectively), for the SAXS SAXS whichinterlamellar were clearly reflec differenttion. In any from case, those X determinedc was again forthe close interlamellar to 0.81; (c) reflection. The pattern Inany and case, correlationXc wasfunction again of close the sample to 0.81; after (c) The being pattern cooled and to correlation room temperature function from of the the sample melt afterstate beingwere similar cooled toto roomthose temperaturedetermined from from the the initial melt statesample were and similar indicated to those a similar determined lamellar fromorganization. the initial Nevertheless, sample and indicateda slight decrease a similar of lamellar lc was organization. detected (i.e., Nevertheless, 5.3 nm as opposed a slight decrease to 6.1 nm) of lc aswas well detected as a decrease (i.e., 5.3 nm of ascrystallinity opposed to within 6.1 nm) lamellar as well stacks as a decrease (i.e., 0.78 of crystallinityas opposed withinto 0.80), lamellar in agreement stacks (i.e., with 0.78 the as lack opposed of an toannealing 0.80), in process agreement for withthe melt the lackcrystallized of an annealing sample. process for the melt crystallized sample.

3.5.3.5. Evolution of Morphologic Parameters duringduring MeltMelt CrystallizationCrystallization ofof Poly(p-dioxanone)Poly(p-dioxanone) SamplesSamples Figure 12 illustrates the the correlation correlation functions functions from from patterns patterns taken taken during during the the cooling cooling run run (2 ◦ (2°C/min)C/min) of ofa melted a melted PDO PDO sample. sample. It Itis isclear clear that that LLγ,γ l,al, aand, and lcl decreasedc decreased progressively progressively and and had similar valuesvalues (7.6,(7.6, 1.5,1.5, andand 6.16.1 nm)nm) toto thosethose observedobserved forfor thethe initialinitial granulatedgranulated PDOPDO sample.sample. The decrease inin lamellarlamellar thicknessthickness cancan bebe duedue to the lower value expected when crystallization temperature decreases andand alsoalso to to a lamellara lamellar insertion insertion mechanism mechanism (i.e., (i.e., formation formation of thinner of thinner lamellar lamellar crystals crystals between between loosely stackedloosely primarystacked primary lamellae). lamellae). Similar results Similar were results obtained were atobtained different at cooling different rates, cooling but it rates, is remarkable but it is thatremarkable morphological that morphological parameters wereparameters practically were identical practically at room identical temperature, at room temperature, as shown in Figureas shown 12 forin Figure the sample 12 for cooled the sample at 10 ◦cooledC/min. at 10 °C/min.

Figure 12. Correlation functions of patterns obtained at the indicated temperatures during the cooling runrun (2(2◦ C/min)°C/min) from from the the melt melt state. state. The correlationThe correlati functionon function of the pattern of the obtained pattern at obtained room temperature at room aftertemperature cooling atafter 10 ◦coolingC/min at is 10 also °C/min shown is foralso comparative shown for comparative purposes. purposes.

Note that crystallization took place at lower temperatures when the cooling rate was increased, Note that crystallization took place at lower temperatures when the cooling rate was increased, and consequently lower lamellar thicknesses should be expected. Therefore, the invariance observed and consequently lower lamellar thicknesses should be expected. Therefore, the invariance observed for lc suggests a counterbalance effect derived from the enhanced insertion mechanism (i.e., decrease for lc suggests a counterbalance effect derived from the enhanced insertion mechanism (i.e., decrease of lamellar thickness) when the cooling rate was decreased. of lamellar thickness) when the cooling rate was decreased.

3.6.3.6. Changes in Microstructure ofof Poly(p-dioxanone)Poly(p-dioxanone) DegradedDegraded SamplesSamples duringduring HeatingHeating InsightsInsights on the the crystalline crystalline microstructure microstructure of of su suturestures can can be be obtained obtained following following the the evolution evolution of ofSAXS SAXS patterns patterns of ofdegraded degraded samples samples during during a asubseq subsequentuent heating heating process process [25]. [25]. To To this end, PDOPDO suturessutures werewere exposedexposed toto hydrolytichydrolytic degradationdegradation media at pHs 7 and 11 andand atat 3737 ◦°CC for 3636 daysdays toto analyze samples with clear differences in their degree of hydrolysis. Specifically, Mw values of 117,700 analyze samples with clear differences in their degree of hydrolysis. Speciﬁcally, Mw values of 117,700 and 83,800 g/mol were determined after exposure to pHs 7 and 11, respectively. Weight losses were close to 3% (pH 7) and 11% (pH 11). Micrographs in Figures 13 and 14 reveal the greater

Polymers 2016, 8, 351 15 of 18

Polymers 2016, 8, 351 15 of 18 Polymers 2016, 8, 351 15 of 18 and 83,800 g/mol were determined after exposure to pHs 7 and 11, respectively. Weight losses were morphologicalclose to 3% (pH changes 7) and 11% occurred (pH 11). under Micrographs basic conditions in Figures 13and and specifically 14 reveal thethe greater appearance morphological of deep morphological changes occurred under basic conditions and specifically the appearance of deep transversalchanges occurred cracks underthat led basic to narrow conditions disks andand specificallytortuous suture the appearance surfaces (Figure of deep 13). transversal This degradation cracks transversal cracks that led to narrow disks and tortuous suture surfaces (Figure 13). This degradation canthat be led interpreted to narrow disksas a consequence and tortuous of suture greater surfaces hydrolysis (Figure of interlamellar 13). This degradation amorphous can regions, be interpreted which can be interpreted as a consequence of greater hydrolysis of interlamellar amorphous regions, which areas a depleted consequence and dissolved of greater hydrolysisin the medium of interlamellar [11]. The morphology amorphous of regions, sutures which exposed are to depleted neutral andpH are depleted and dissolved in the medium [11]. The morphology of sutures exposed to neutral pH fordissolved a relatively in the short medium time [11 is]. quite The morphology different beca ofuse sutures in this exposed case smoother to neutral surfaces pH for a relativelyand numerous short for a relatively short time is quite different because in this case smoother surfaces and numerous longitudinaltime is quite differentcracks formed because (Figure in this case14). smootherIn fact, it surfaces has been and reported numerous that longitudinal hydrated, cracks interfibrillar formed longitudinal cracks formed (Figure 14). In fact, it has been reported that hydrated, interfibrillar (Figureamorphous 14). regions In fact, itswell has more been reportedeasily than that interlam hydrated,ellar interfibrillaramorphous regions amorphous [12]. regionsNote that swell the latter more amorphous regions swell more easily than interlamellar amorphous regions [12]. Note that the latter areeasily constituted than interlamellar by tie chains, amorphous which connect regions the [12 lamellae]. Note that in each the latterfibril arewhereas constituted fewer tie by chains tie chains, are are constituted by tie chains, which connect the lamellae in each fibril whereas fewer tie chains are whichexpected connect to connect the lamellae adjacent in fibrils. each Thus, fibril whereaswater diffusion fewer tie and chains formation are expected of longitudinal to connect cracks adjacent seem expected to connect adjacent fibrils. Thus, water diffusion and formation of longitudinal cracks seem tofibrils. be favored. Thus, water diffusion and formation of longitudinal cracks seem to be favored. to be favored.

Figure 13. SAXS patterns taken at representative temperatures of 25 ◦°C (a); 102 ◦°C (b) and 107 °C◦ (c) FigureFigure 13. 13. SAXSSAXS patterns patterns taken taken at at representative representative temperatures temperatures of 25 °CC( (aa);); 102 102 °CC( (bb)) and and 107 107 °CC( (cc)) during a heating run at 10 ◦°C/min of a PDO suture previously degraded in a pH 11 hydrolytic medium duringduring a a heating heating run run at at 10 10 °C/minC/min of of a aPDO PDO suture suture previously previously degraded degraded in in a a pH pH 11 11 hydrolytic hydrolytic medium medium for 36 days. The pattern obtained at room temperature after cooling (10 ◦°C/min) and the optical forfor 36 36 days. The The pattern pattern obtained obtained at at room room temper temperatureature after cooling (10 °C/min)C/min) and and the the optical optical micrograph of the degraded suture are shown in (d,e), respectively. micrographmicrograph of of the the degraded degraded suture are shown in ( d,e), respectively.

Figure 14. SAXS patterns taken at representative temperatures of 25 °C (a); 102 °C (b) and 112 °C (c) Figure 14. SAXS patterns taken at representative temperatures of 25 °C◦ (a); 102 °C◦ (b) and 112 °C◦ (c) duringFigure a 14. heatingSAXS run patterns at 10 °C/min taken at of representative a PDO suture temperatures previously degraded of 25 C( ina );a 102pH 7 C(hydrolyticb) and 112 mediumC(c) during a heating run at 10 °C/min◦ of a PDO suture previously degraded in a pH 7 hydrolytic medium forduring 36 days. a heating The runpattern at 10 obtainedC/min of at a room PDO suturetemper previouslyature after degraded cooling in(10 a °C/min) pH 7 hydrolytic and the medium optical for 36 days. The pattern obtained at room temperature after cooling (10 ◦°C/min) and the optical micrographfor 36 days. of The the patterndegraded obtained suture are at room shown temperature in (d,e), respectively. after cooling (10 C/min) and the optical micrographmicrograph of of the the degraded degraded suture are shown in ( d,e), respectively. SAXS patterns of degraded samples showed a slight decrease of meridional interlamellar SAXS patterns of degraded samples showed a slight decrease of meridional interlamellar spacing,SAXS LB patterns, because of of degraded the more samplescompact showed structure a slight achieved decrease after ofscission meridional of chains interlamellar belonging spacing, to the spacing, LB, because of the more compact structure achieved after scission of chains belonging to the Lamorphous, because ofregions. the more Therefore, compact the structure initial achievedspacing of after 9.6 scissionnm decreased of chains to 9.2 belonging and 9.3 tonm the after amorphous 36 days amorphousB regions. Therefore, the initial spacing of 9.6 nm decreased to 9.2 and 9.3 nm after 36 days ofregions. degradation Therefore, in basic the initial and neutral spacing pH, of 9.6 respectively. nm decreased Analysis to 9.2 and of 9.3correlation nm after functions 36 days of (not degradation shown) of degradation in basic and neutral pH, respectively. Analysis of correlation functions (not shown) indicatedin basic and that neutral lc and pH, la values respectively. decreased Analysis from of 6. correlation1 to 1.5 nm, functions respectively, (not shown) to 5.3 indicatedand 1.3 nm that forlc andthe indicated that lc and la values decreased from 6.1 to 1.5 nm, respectively, to 5.3 and 1.3 nm for the basic pH. Logically, SAXS crystallinity increased during degradation since the main change occurred basic pH. Logically, SAXS crystallinity increased during degradation since the main change occurred in the amorphous layer. in the amorphous layer.

Polymers 2016, 8, 351 16 of 18

la values decreased from 6.1 to 1.5 nm, respectively, to 5.3 and 1.3 nm for the basic pH. Logically, SAXS crystallinity increased during degradation since the main change occurred in the amorphous layer. Lamellar crystals in degraded samples were able to recrystallize and even reorient during subsequent heating runs in an easier way than that observed for the initial suture. The increased freedom resulting from scission of tie chains belonging to interfibrillar and interlamellar amorphous regions should play a fundamental role. Diffraction patterns during subsequent heating showed clear differences between highly and scarcely degraded samples. Thus, in the first case, an increase in lamellar thickness, together with the appearance of an intense equatorial reflection associated with the interfibrillar spacing previous to the disappearance of the meridional reflection, was observed. Specifically, LB values of 13.2 and 11.7 nm were determined at temperatures close to fusion (i.e., 102 ◦C). This behavior was similar to that observed for the initial suture taking into account the differences in spacings and intensities of reflections. Logically, greater spacings and intensities were detected for degraded samples as a consequence, in the first case, of an enhanced reorganization when tie interconnecting chains were cleaved and, in the second case, of a higher contrast between amorphous/crystalline regions. Figure 14 reveals a different evolution when samples were degraded in the pH 7 medium because the great thickening of lamellae was hindered due to a still compact chain packing (note that weight loss was minimal). Therefore, the thickened lamellae (LB = 11.4 nm) tilted with respect to the fiber axis gave rise to a second reflection. The breadth of reorganized crystals was also clearly increased, as could be deduced from the observed spot like reflections. Finally, Figures 13 and 14 show the reversibility of the thermal process for degraded samples since the diffraction patterns obtained after cooling to room temperature are identical to those obtained from the initial sample before any thermal treatment.

4. Conclusions PDO showed complex melting and crystallization peaks because of a typical lamellar thickening process and the existence of different nucleation mechanisms, respectively. Calorimetric analysis of non-isothermal crystallization showed an increase of the Avrami exponent at low cooling rates, which could be associated with a homogeneous nucleation process instead of the instantaneous nucleation observed at high rates. Isoconversional analyses from non-isothermal calorimetric data revealed the existence of two crystallization regimes, demonstrating the suitability of this methodology. These regimes were well characterized by optical microscopy observations, and the non-isothermal crystallization results were in relatively good agreement with those previously reported from isothermal studies. Real time SAXS proﬁles taken during heating and cooling processes showed the occurrence of a lamellar reordering process and a lamellar insertion mechanism that led to an increase and a decrease in lamellar thickness, respectively. SAXS patterns taken during heating of samples degraded under neutral and basic pH had a different evolution that revealed the existence of interlamellar and interﬁbrillar amorphous domains.

Acknowledgments: The authors acknowledge support from MINECO and FEDER (MAT2015-69547-R), and the Generalitat de Catalunya (2014SGR188). Diffraction experiments were performed at NCD beamline at ALBA Synchrotron with the collaboration of ALBA staff. Author Contributions: All authors made contributions to the development of this manuscript and approved the final manuscript. Yolanda Márquez contributed to the experimental work, Lourdes Franco to calorimetric analyses, Juan Carlos Martínez assisted with synchrotron analyses, Pau Turon contributed to the concept and revision of the manuscript, and Jordi Puiggalí contributed to the concept, analyses of data, and design and revision of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Polymers 2016, 8, 351 17 of 18

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