Silane-Treated Basalt Fiber–Reinforced Poly(Butylene Succinate) Biocomposites: Interfacial Crystallization and Tensile Properties

polymers

Article Silane-Treated Basalt Fiber–Reinforced Poly(butylene succinate) Biocomposites: Interfacial Crystallization and Tensile Properties

Lin Sang 1, Mingyuan Zhao 1, Qiushi Liang 1 and Zhiyong Wei 2,* ID

1 School of Automotive Engineering, State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116024, China; [email protected] (L.S.); [email protected] (M.Z.); [email protected] (Q.L.) 2 Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China * Correspondence: [email protected]; Tel.: +86-411-8498-6463

Received: 12 July 2017; Accepted: 7 August 2017; Published: 9 August 2017

Abstract: In this work, an economical modifier silane agent—KH550—was used for surface treatment of basalt fiber. Then, a biodegradable poly(butylene succinate) (PBS)/modified basalt fiber (MBF) biocomposite was successfully developed. The effects of silane treatment and fiber mass content on crystalline structure, isothermal crystallization process and mechanical performance of composites were evaluated. The interfacial crystallization of PBS on the surface of MBF was investigated by using a polarized optical microscope (POM). The transcrystalline (TC) structure could be clearly observed and it grew perpendicular to the surface of MBF, which boosted the nucleation ability on PBS crystallization and the strong interfacial interaction between PBS and silane-treated basalt fiber. Under isothermal crystallization kinetics, the incorporation of basalt fiber enhanced the crystallization rate and reduced the crystallization half-time values of composites compared with that of neat PBS due to a heterogeneous nucleation effect. Furthermore, tensile results confirmed that the presence of MBF could greatly improve the tensile strength and modulus. The predicted interfacial shear strength (IFSS) suggested that an enhancement of interfacial bonding could be realized via interfacial crystallization, which was also verified by SEM images. The PBS/MBF biocomposites can be applied in many fields as a low-cost, lightweight, and biodegradable composite material.

Keywords: composites; crystal structure; mechanical properties; structure-property relations

1. Introduction In recent decades, considering environmental issues and the energy crisis, green biodegradable polymeric materials and composites have received more and more attention. Basalt fiber–reinforced plastics (BFRPs) are newly emerging composite materials due to their being recyclable, cost effective, and lightweight [1–4]. Basalt fiber (BF), produced by abundant natural basalt rocks, is a promising fiber reinforcement, which endows basalt fiber with excellent chemical, weather, and heat resistance [5,6]. Although it possesses similar compositions to glass fiber (GF), basalt fiber exhibits better strength and higher elastic modulus characteristics. Among basalt fiber–reinforced thermoplastics, poly(butylene succinate) (PBS) is an excellent green plastics polymerized from bio-based renewable resources monomers butanediol and succinic acid. Moreover, PBS possesses a variety of desirable properties including biodegradability, renewability, and chemical resistance [7–9]. However, few investigations have been performed on the compatibility between PBS matrix and bare BF, which limits the practical application of basalt fiber reinforced PBS composites [10].

Polymers 2017, 9, 351; doi:10.3390/polym9080351 www.mdpi.com/journal/polymers Polymers 2017, 9, 351 2 of 14

Various efforts have been made to improve the fiber/matrix interfacial interaction, including functionalization of matrix, adding compatibilizer, and surface modification of the fibers [11–13]. These methods could improve interfacial adhesion, contributing to a certain extent to an efficient stress transferring from the matrix to fibers. Compared with the functionalization of the matrix or using a compatibilizer [11], surface modification of fibers is the most common and simplest method to obtain interfacial interaction. Silane treatment [12,13] is an effective, commercially available, and low cost method for surface modification of fibers without destroy the structure of the fibers themselves or weaken fiber strength. In fiber-reinforced crystalline or semi-crystalline polymer composites, crystallization behavior has been considerably researched, as it affects both the crystalline structure and crystallization behavior and also influences the mechanical performance of composites. Specially, the development of interfacial crystallization occurring in the interface between fiber and matrix would enhance interfacial adhesion [14–20]. It is generally accepted that the interfacial crystallization would substantially influence mechanical behavior [21–24]. Some authors have reported that transcrystalline of matrix polymer (i.e., poly(lactic acid) (PLA) [21], isotatic polypropylene (iPP) [22], or PBS [23]) exhibited a positive effect on the interfacial and mechanical properties. Wang et al. [25] studied transcrystalline effects on sisal fiber-reinforced PLA composites. Zhou et al. [9] directly introduced dopamine as a coupling agent to treat ramie fiber and investigated the interfacial crystallization of PBS. In this literature, interfacial crystallization with different microstructures and morphologies were extensively investigated, and interfacial shear stress (IFSS) was discussed from the point of a single fiber. However, few studies have focused on investigating the effect of interfacial crystallization on mechanical properties in short fiber–reinforced composites at a certain ratio. In the present paper, we first treated basalt fibers using the economical silane agent KH550 as a modifier and then fabricated PBS/untreated fiber and PBS/modified basalt fiber biocomposites with varying ratios by internal mixer and the injection molding method. The growth of a transcrystalline structure on the surface of silane-treated basalt fiber was observed by polarized optical microscopy (POM), and the isothermal crystallization kinetics of PBS and its composites were analyzed by differential scanning calorimetry (DSC). The effect of silane treatment and fiber loading on the tensile properties of PBS/MBF biocomposites were compared, and tensile fractured topography was observed by scanning electron microscope (SEM). Moreover, a mathematical model was adopted to predict the interfacial shear strength (IFSS), which would provide useful information on the relationship between interfacial strength and mechanical performance.

2. Materials and Methods

2.1. Materials Poly(butylene succinate) (PBS, Anqing Hexing Chemical, Ltd., Anqing, China) is a thermoplastic polymer with a melting ﬂow index (MFI) of 15–25 g·10 min−1 (150 ◦C, 2.16 kg) and a density of 1.26 g·cm−3. Its melting point is 113 ◦C, and its number-average molecular weight is 1.7 × 105 g·mol−1, as reported by the manufacture. The continuous basalt ﬁbers (GBF 9-800, Shanxi Basalt Fiber Technology Co., Ltd., Taiyuan, China) possess the following characteristics: their density is 2.8 g·cm−3, and their tensile strength and tensile modulus is 3.0 and 90–110 GPa, respectively. The silane coupling agent KH550 (NH2CH2CH2CH2Si(OC2H5)3), acetone, ethanol (95% purity), and other chemicals used in this study were purchased from Kelong Chemical reagent Ltd., Chengdu, China.

2.2. The Silane Treatment of Bbasalt Fiber (BF) The surface treatment of BF was performed according to previous work [26]. Basalt fibers were first cleaned with acetone/ethanol for 2 h to remove the organic residues from the fiber surfaces. The fibers were repeatedly washed with deionized water to neutrality. After that, BF was immersed Polymers 2017, 9, 351 3 of 14 in the silane coupling agent (KH550, 0.75 wt %) in water/ethanol for 30 min. Finally, the fibers were reacted and dried in a vacuum oven at 120 ◦C for 2 h. The silane-treated fibers were marked as MBF.

2.3. Preparation of PBS/MBF Biocomposites The continuous untreated and silane-treated basalt fibers were cut into a settled length of 10 mm. Basalt fibers and PBS granules were dried in a vacuum oven at 80 ◦C for 12 h to before blending. PBS and silane-treated basalt fibers were compounded via a two-roll mill method. The blending was conducted via the following procedure: First, about half of the PBS was placed inside the mixing chamber at 130 ◦C for about 1 min at 30 rpm. Then, varying BF (or MBF) content was added over a period of 3 min. Then, the rest of PBS resin was introduced into the mixing chamber. The mixing speed was increased to 60 rpm, and mixing continued for another 8 min. The melting composites were cooled and subsequently chopped to granules. The resulted granules were dried at 85 ◦C overnight and then molded using an injection molding machine (XTK 1200, Xiatian General Machinery, Ningbo, China). The temperatures at the three injection regions were 140 ◦C, 155 ◦C and 155 ◦C, respectively, and the nozzle temperature was 145 ◦C. The fiber contents were set at 2, 5, 10, and 15 wt %, respectively. According to the different loadings of basalt fiber, the injection-molded specimens reinforced with untreated and silane-treated basalt fibers were marked as PBS/MBF (98/2), PBS/BF (95/5), PBS/MBF (95/5), PBS/MBF (90/10), and PBS/MBF (85/15), respectively. PBS/BF (95/5) were prepared and set as control sample.

2.4. Characterization

2.4.1. Scanning Electron Microscope (SEM) The surface morphology of basalt ﬁber and tensile fractured surface of composites were examined using a ﬁeld emission scanning electron microscope (S-4800N-FESEM, Zeiss, Oberkochen, Germany). The specimens were pre-coated with a thin gold layer before observation.

2.4.2. X-ray Photoelectron Spectrometer (XPS) The possible interaction between the ﬁber and silane agent was examined using an X-ray photoelectron spectrometer (XPS, ESCALABTM 250Xi, Thermo Fisher Scientiﬁc, Waltham, MA, USA) focused monochromatized Al Kα radiation (15 kV). The survey spectra were collected over a wide binding energy range from 0 to 1350 eV at a take-off angle of 45◦. High resolution spectra of C1s peak were specially recorded.

2.4.3. Polarized Optical Microscope (POM) The dynamic process of interfacial crystallization morphology during isothermal crystallization on the surface of MBF were in situ observed by a polarized optical microscope (POM, Leica DM4500P, Heidelberg, Germany), which was equipped with a (Linkam THMS600, Linkam Scientiﬁc instruments, Tadworth, UK) hot-stage and a digital camera (Leica CCD, Heidelberg, Germany). Composites with varying BF loading were made into sandwiches between two cover slips. Then, they were heated to 130 ◦C, kept at this temperature for 5 min to allow complete melting, and quickly quenched to 89 ◦C for isothermal crystallization.

2.4.4. Differential Scanning Calorimetry (DSC) Isothermal crystallization behavior of PBS, PBS/BF and PBS/MBF composites were performed in a nitrogen atmosphere on a differential scanning calorimeter (DSC1, Mettler-Toledo, Zurich, Switzerland). The samples with 6–8 mg were melted at 130 ◦C for 5 min to eliminate the previous thermal history and then quick-cooled at a rate of 60 ◦C·min−1 to a series of isothermal crystallization temperatures (Tc). The Tc was held for a period of time to ensure the completion of crystallization. The heat ﬂows during the crystallization process were recorded for data analysis. Polymers 2017, 9, 351 4 of 14 Polymers 2017, 9, 351 4 of 14

2.4.5. Tensile Properties Tensile propertiesproperties werewere measuredmeasured accordingaccording toto GB/T1040-92GB/T1040-92 standard using a universal tester (Sans UTM5105X,UTM5105X, Shenzhen,Shenzhen, China). China). The The tensile tensile specimens specimens were were carried carried out out at room at room temperature temperature (RT) and(RT) aboutand about 50% 50% relative relative humidity humidity with with a 10 a kN10 kN load load cell cell and and a cross-heada cross-head speed speed of of 5 5 mm mm·min·min−−1.. The average values of tensile parameters of PBS and its composites were calculated from ﬁvefive successful samples.

3. Results

3.1. Modification Modification of Basalt Fiber The modification modification process process is isphotogra photographicallyphically shown shownin Figure in Figure1a. KH5501a. KH550 (NH CH CH CH Si(OC H ) ), an economical organo-silane, was adopted in this study. (NH2CH2CH22CH2Si(OC2 2H25)3), an2 economical5 3 organo-silane, was adopted in this study. Silane Silanetreatment treatment has proved has to proved be a low-cost to be a and low-cost efficien andt method efficient to endow method the to fiber-matrix endow the an fiber-matrix improved aninterfacial improved bonding interfacial [26–28]. bonding It is clearly [26–28 visible]. It is that clearly raw visiblebasalt fibers that rawwere basalt brownish fibers in were color, brownish whereas inthe color, modified whereas basalt the fiber modified (MBF) displayed basalt fiber a (MBF)light brow displayednish color. a light During brownish silane color.treatment, During the silane treatment,agent first thehydrolyzed, silane agent formed first hydrolyzed, reactive silanol, formed and reactive then silanol, adsorbed and thenand adsorbedcondensed and on condensed the fiber on the fiber surface. Furthermore, the functional –NH groups could be connected with the –COOH surface. Furthermore, the functional –NH2 groups could2 be connected with the –COOH end groups endof PBS groups by covalent of PBS bybonds. covalent Thus, bonds. after silane Thus, treatment, after silane efficient treatment, interfacial efficient bonding interfacial between bonding the betweenreinforcing the element reinforcing (BF) and element the polymeric (BF) and thematrix polymeric (PBS) was matrix expected (PBS) to was have expected been established. to have been To established.confirm the modification To confirm the of modificationthe basalt fibers, of the SEM basalt was fibers,conducted SEM on was the conducted pristine and on themodified pristine basalt and modifiedfibers. As basaltshown fibers. in Figure As shown 1b,c, a in rough Figure surface1b,c, a roughwith small surface residues with small was found residues onwas the foundMBF surface, on the MBFwhereas surface, a clean whereas and smooth a clean andsurface smooth was surface observed was on observed pristine on basalt pristine fiber. basalt This fiber. indicated This indicated that the thatsilane the agent silane KH550 agent successfully KH550 successfully interacted interacted on the fiber on the surfaces. fiber surfaces.

Figure 1. (a) The modification process of BF; (b,c) SEM micrograph of BF and MBF. Figure 1. (a) The modification process of BF; (b,c) SEM micrograph of BF and MBF. Prior to compounding, XPS was used to detect the elements and interactions after silane treatmentPrior toof compounding,the basalt fibers. XPS As was shown used toin detectFigure the 2a, elements the XPS andwide interactions scan spectra after revealed silane treatment that the ofpresence the basalt of carbon fibers. and As oxygen, shown inand Figure a slight2a, theamount XPS of wide nitrogen scan spectracould be revealed detected that on the surface presence of ofBF carbonand MBF. and For oxygen, MBF samples, and a slight the amountrelative ofamount nitrogen of detected could be oxygen detected was on increased, the surface which of BF most and MBF.probably For MBForiginated samples, from the the relative chemical amount compositio of detectedn of oxygensilane agent. was increased, Moreover, which the mostrecorded probably high originatedresolution spectra from the of chemical the carbon composition element were of silane evaluated agent. and Moreover, curve-fitted. the recorded The detailed high resolution chemical spectraanalysis of of the C1s carbon high elementresolution were scan evaluated spectra of and BF curve-fitted. and MBF are The illustrated detailed chemicalin Figure analysis 2b,c. The of C1s highspectra resolution of the MBF scan can spectra be curve-fitted of BF and MBFwith arethree illustrated peaks. Beside in Figure the 2aliphaticb,c. The C1sC–C/C–H spectra peak of the at MBF284.8 eV (taken also for charge shift referencing) [29], one is at binding energy (BE) of 286.3 eV for –C–O species and the other is at BE of 288.8 eV for the –C=O species. These rational-derived peaks of C1s

Polymers 2017, 9, 351 5 of 14 can be curve-ﬁtted with three peaks. Beside the aliphatic C–C/C–H peak at 284.8 eV (taken also for chargePolymers shift 2017 referencing), 9, 351 [29], one is at binding energy (BE) of 286.3 eV for –C–O species and the5 other of 14 is at BE of 288.8 eV for the –C=O species. These rational-derived peaks of C1s attributed to different chemicalattributed environment to different implychemical that environment the silane treatment imply onthat BF the surface silane was treatment successfully. on BF surface was successfully.

(a) BF (b) BF (c) MBF C1s MBF O1s C1s C1s

-CH2- N1s

C-O Counts / s Counts / s -C=O Intensity (a.u.) Intensity

700 600 500 400 300 200 100 0 290 288 286 284 282 280 290 288 286 284 282 280 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV)

Figure 2.2. (a(a)) X-ray X-ray photoelectron photoelectron spectrometer spectrometer (XPS) (XPS) wide wide scan spectrascan spectra of basalt of fiberbasalt (BF) fiber and (BF) modified and basaltmodified fiber basalt (MBF) fiber with (MBF) labeled with photoelectron labeled photoelect (C1s, O1s,ron and(C1s, N1s), O1s, and and XPS N1s), high and resolutions, XPS high resolutions, C1s spectra ofC1s (b spectra) BF and of ( c()b MBF.) BF and (c) MBF.

3.2. MBF-Induced Transcrystallization (TC)(TC) ofof PBSPBS In orderorder toto investigate investigate the the effect effect of fiberof fiber treatment treatment on PBS on crystallinePBS crystalline morphology, morphology, polarized polarized optical microscopeoptical microscope (POM) images(POM) ofimages crystalline of crystalline structure structure at the interface at the betweeninterface PBSbetween and MBF PBS areand presented MBF are inpresented Figure3 in. The Figure isothermal 3. The isothermal crystallization crystallization temperature temperature was set atwas 89 set◦C at for 89 °C 35 for min. 35 Formin. neat For PBS,neat severalPBS, several well-developed well-developed spherulites spherulite withs diameterswith diameters in a range in a of range 70–100 ofµ m70–100 were μ clearlym were observed. clearly Figureobserved.3b–f Figure depicted 3b–f the depicted PBS spherulites the PBS ofspherulites composites of in composites the presence in ofthe untreated presence and of untreated silane-treated and basaltsilane-treated fiber. For basalt the same fiber. crystallization For the same time, crystall moreization crystals time, were more observed crystals in the were composites observed than in the neatcomposites PBS, which than indicatedthe neat PBS, that whic the presenceh indicated of basalt that the fiber presence played of an ba acceleratingsalt fiber played role in an the accelerating nucleation ofrole PBS. in the For nucleation composites of reinforced PBS. For withcomposites pristine reinforced BF, no crystallization with pristine of BF, PBS no on crystallization the untreated BFof surfacePBS on wasthe untreated observed BF (Figure surface3c). was On observed the other (Figure hand, for3c).PBS/MBF On the other system, hand, a for special PBS/MBF transcrsytalline system, a special (TC) structuretranscrsytalline was successfully (TC) structure induced was on successfully the surface ofindu silane-treatedced on the fiberssurface (as of shown silane-treated in Figure 3fibersb). Also, (as theshown formation in Figure of 3b). TC structureAlso, the wasformation prior toof regularTC structure PBS spherulites, was prior to since regular some PBS regular spherulites, spherulites since weresome extremelyregular spherulites small. In contrast,were extremely the crystallized small. In size contrast, of spherulites the crystallized in PBS/BF size (95/5) of spherulites composites in wasPBS/BF relatively (95/5) composites uniform. This was phenomenon relatively uniform. indicated This that phenomenon the surface ofindicated the silane-treated that the surface basalt of fibers the showedsilane-treated improved basalt heterogeneous fibers showed nucleation improved abilityheterogeneous for PBS, whichnucleation can induce ability thefor molecularPBS, which chain can toinduce crystallize the molecular into orderly chain structure. to crystallize However, into atorderly a fixed structure. crystallization However, temperature, at a fixed a highercrystallization density oftemperature, TC structure a higher was obtaineddensity of at TC lower structure BF loading was obtained (i.e., 2 at wt lower % and BF 5loading wt %), (i.e., while 2 wt a % mixture and 5 wt of spherulites%), while a andmixture TC structureof spherulites was found and TC in structure higher BF was loading found (10 in wt higher % and BF 15 loading wt %). (10 With wt increasing % and 15 amountswt %). With of silane-treated increasing amounts fibers, theof heterogeneoussilane-treated fibers, nucleation the heterogeneous effect was expected nucleation to be effect enhanced, was whichexpected accelerated to be enhanced, the formation which ofaccelerated spherulites. the On formation the other of hand, spherulites. the number On the of other nucleation hand, sites the wasnumber also of increased, nucleation which sites inducedwas also an increased, interfacial which crystallization induced an structure. interfacial Therefore, crystallization one may structure. obtain differentTherefore, crystalline one may structuresobtain different in silane-treated crystalline fiber structures reinforced in silane-treated PBS composites fiber by reinforced controlling PBS the modifiedcomposites fiber by loading.controlling the modified fiber loading. Figure 44 showed showed thethe nucleationnucleation andand growth growth of of the the transcrystalline transcrystalline structure structure of of the the PBS/MBF PBS/MBF (98/2) composite composite as as a a representative representative sample. sample. As As shown shown in in Figure Figure 44a,a, brightbright interfacialinterfacial TCTC structurestructure was gradually formed formed as as the the crystallization crystallization time time increased increased (10 (10 min min →→ 4545 min). min). After After nucleation nucleation on onthe thesurface surface for 10 for min, 10 interfacial min, interfacial crystallization crystallization morphology morphology was observed was observedand identified. and identified.However, However,almost no almost PBS spherulites no PBS spherulites were observed were observed far aw faray away from from the thesilane-treated silane-treated fiber. fiber. When When the crystallization time increased to 15 min, the transcrystallinetranscrystalline (lamellae) structure began to grow perpendicular toto the the silane-treated silane-treated fiber fiber surface, surf andace, the and TC structurethe TC wasstructure formed. was As theformed. crystallization As the timecrystallization prolonged, time the prolonged, growth of lamella the growth was restrictedof lamella along was restricted the long axis along of fiber,the long and axis the TCof fiber, thickness and the TC thickness was further increased. By plotting crystallization time and crystalline size, almost a linear relationship can be observed, as shown in Figure 4b. The TC layer on the silane-treated basalt fiber grew to about 8 μm after 10 min and gradually expanded into 87 μm after 45 min. Additionally, the crystalline structure of PBS in the presence of BF is recorded in Supporting Information (Figure S1) during the crystallization process. Comparing the formation processes of crystalline structures, it is

Polymers 2017, 9, 351 6 of 14 was further increased. By plotting crystallization time and crystalline size, almost a linear relationship can be observed, as shown in Figure4b. The TC layer on the silane-treated basalt ﬁber grew to about 8 µm after 10 min and gradually expanded into 87 µm after 45 min. Additionally, the crystalline structure of PBS in the presence of BF is recorded in Supporting Information (Figure S1) during the Polymers 2017, 9, 351 6 of 14 crystallizationPolymers 2017 process., 9, 351 Comparing the formation processes of crystalline structures, it is predictable6 of 14 thatpredictable the presence that ofthe MBF presence should beof favorableMBF should in heterogeneous be favorable nucleation in heterogeneous and transcrystalline nucleation growth and predictable that the presence of MBF should be favorable in heterogeneous nucleation and comparedtranscrystalline with thatgrowth of BF. compared with that of BF. transcrystalline growth compared with that of BF.

FigureFigure 3. 3.Polarized Polarized opticaloptical microscope (POM) (POM) images images of ( ofa) PBS, (a) PBS, (b) PBS/MBF (b) PBS/MBF (98/2), (98/2),(c) PBS/BF (c) (95/5), PBS/BF Figure 3. Polarized optical microscope (POM) images of (a) PBS, (b) PBS/MBF (98/2), (c) PBS/BF (95/5), (95/5),(d) PBS/MBF (d) PBS/MBF (95/5), (95/5),(e) PBS/MBF (e) PBS/MBF (90/10), and (90/10), (f) PBS/MBF and (f) (85/15) PBS/MBF at 89 (85/15) °C for 35 at min. 89 ◦C for 35 min. (d) PBS/MBF (95/5), (e) PBS/MBF (90/10), and (f) PBS/MBF (85/15) at 89 °C for 35 min.

Figure 4. (a) The growth of transcrystallization (TC) on MBF of PBS/MBF (98/2) composites crystallized at 89 °C with different time; (b) thickness of TC layer; (c) schematic illustration of TC structure. Figure 4. (a) The growthgrowth ofof transcrystallizationtranscrystallization (TC)(TC) onon MBF MBF of of PBS/MBF PBS/MBF (98/2)(98/2) composites crystallized at 89Figure ◦°CC with 4c illustrates different time; the (evolutionb)) thicknessthickness of ofof TC TCTC stru layer;layer;cture ((cc)) schematicschematicat the interface illustrationillustration between ofof TCTC the structure.structure. fiber and PBS matrix. The nuclei sites on the surfaces of silane-treated basalt fiber were sufficiently high and distributedFigure 4c randomly, illustrates thusthe evolutionthe initiated of TCcrystallization structure at fromthe interface the nuclei between and the the development fiber and PBS matrix.perpendicular The nuclei to thesites fibre on direction the surfaces occurred of silanesimult-treatedaneously. basalt When fiber the crystallizedwere sufficiently time increased, high and distributedthe spherulites randomly, impinged thus each the other initiated and grewcrystallization only in one from direction the nucleidue space and confinement the development and perpendicularfinally developed to the into fibre a directiontranscrystalline occurred region. simult Theaneously. formation When of TC the structure crystallized has proventime increased, to be thebeneficial spherulites to enhance impinged the interfacialeach other interaction and grew by on contributingly in one direction to effective due stress space transforming confinement from and finallythe matrix developed to the intofiber ainside transcrystalline [9,22]. In addition, region. it Thehas beenformation reported of thatTC structurea rough fiber has surface proven with to be beneficial to enhance the interfacial interaction by contributing to effective stress transforming from the matrix to the fiber inside [9,22]. In addition, it has been reported that a rough fiber surface with

Polymers 2017, 9, 351 7 of 14

Figure4c illustrates the evolution of TC structure at the interface between the fiber and PBS matrix. The nuclei sites on the surfaces of silane-treated basalt fiber were sufficiently high and distributed randomly, thus the initiated crystallization from the nuclei and the development perpendicular to the fibre direction occurred simultaneously. When the crystallized time increased, the spherulites impinged each other and grew only in one direction due space confinement and finally developed into a transcrystalline region. The formation of TC structure has proven to be beneficial to enhance the interfacial interaction by contributing to effective stress transforming from the matrix to the fiber inside [9,22]. In addition, it has been reported that a rough fiber surface with low surface energy providesPolymers 2017 abundant, 9, 351 heterogeneous nucleation sites to induce transcrystallinity in7 crystalline of 14 polymers [30,31]. low surface energy provides abundant heterogeneous nucleation sites to induce transcrystallinity in 3.3. Isothermalcrystalline Crystallization polymers [30,31]. Behaviors

Differential3.3. Isothermal scanning Crystallization calorimetry Behaviors (DSC) was further used to monitor the overall isothermal crystallization process of PBS and its composites at varying crystallization temperatures. By adopting Differential scanning calorimetry (DSC) was further used to monitor the overall isothermal T ◦ approximatecrystallization crystallization process of PBS time, and the its compositesc interval at of varying PBS and crystallization its composites temperatures. was 72–78 By adoptingC. To erase previousapproximate thermal history, crystallization all composites time, the T werec interval quick-cooled of PBS and to its the composites targeted was crystallization 72–78 °C. To temperature erase (Tc). Theprevious DSC crystallizationthermal history, exotherms all composites of neat were PBS, quick-cooled PBS/BF, and to PBS/MBFthe targeted composites crystallization are shown in Figuretemperature5. As expected, (Tc). The it DSC is observed crystallization that asexothermsTc increased, of neat PBS, the exothermalPBS/BF, and PBS/MBF peak became composites broader for all theare samples, shown suggestingin Figure 5. As a decreasedexpected, it crystallizationis observed that as rate Tc increased, and an increased the exothermal time forpeak crystallization became completion.broader Onfor theall the other samples, hand, suggesting the crystallization a decreased ratecrystallization of PBS was rate obviouslyand an increased enhanced time for with the crystallization completion. On the other hand, the crystallization rate of PBS was obviously enhanced increased amount of reinforced ﬁber. with the increased amount of reinforced fiber. (a) (b) (c)

d d d c c c o o a) 72oC a) 72 C a) 72 C b o b o b) 74oC b b) 74 C b) 74 C o o

Heat flow (mW) Endo up c) 76 C c) 76oC c) 76 C a Heat flow (mW) upEndo o

a Heat flow (mW)Endo up o a d) 78 C d) 78oC d) 78 C

0 3 6 9 121518 0 3 6 9 12 15 18 0369121518 Time(min) Time(min) Time(min) (d) (e) (f)

d d d c c o

c o a) 72oC a) 72 C b a) 72 C b o b o o b) 74 C b) 74 C b) 74 C o o a o c) 76 C c) 76 C c) 76 C o Heat flow (mW) Endo up (mW) flow Heat a o Heatflow (mW) Endo up o Heat flow (mW) Endo up d) 78 C a d) 78 C d) 78 C

0 3 6 9 12 15 18 0 3 6 9 12 15 18 0369121518 Time(min) Time(min) Time(min) Figure 5. Differential scanning calorimetry (DSC) thermograms of (a) PBS, (b) PBS/MBF (98/2), (c) Figure 5. Differential scanning calorimetry (DSC) thermograms of (a) PBS, (b) PBS/MBF (98/2), PBS/BF (95/5), (d) PBS/MBF (95/5), (e) PBS/MBF (90/10), and (f) PBS/MBF (85/15) at different (c) PBS/BFcrystallization (95/5), (temperatures.d) PBS/MBF (95/5), (e) PBS/MBF (90/10), and (f) PBS/MBF (85/15) at different crystallization temperatures. The well-known Avrami equation was used to track the isothermal crystallization kinetics and ThePBS well-known and its composites, Avrami which equation contribute was used to an to understanding track the isothermal of the evolution crystallization of crystallinity. kinetics The and PBS equation is listed as follows [32,33]: and its composites, which contribute to an understanding of the evolution of crystallinity. The equation is listed as follows [32,33]: 1− =exp− (1) n where Xt is the relative crystallinity, t is1 −theX crystallizationt = exp(−kt time,) n is the Avrami exponent (which is (1) dependent on the mechanism of nucleation and growth geometry), and k is the crystallization rate where constant.Xt is the Equation relative (1) crystallinity, can also be rewrittent is the crystallizationas a double logarithmic time, n formis the as Avramifollows: exponent (which is dependent on the mechanism of nucleation and growth geometry), and k is the crystallization rate ln−ln1− =ln +ln (2) constant. Equation (1) can also be rewritten as a double logarithmic form as follows: Figure 6a illustrates the Avrami plots of relative crystallinity (Xt) versus crystallization time (t) obtained by exotherm data for theln isothermal[− ln (1 − crystallizationXt)] = ln k + at nvariousln t temperatures (PBS/MBF (95/5) (2) as representative examples). The curves of PBS/MBF (95/5) composites showed S-shaped, which was related to nucleation and the growth process. It was also found that the time of isothermal crystallization completion was strongly dependent on Tc. As illustrated in Figure 6b, a linearity in the Avrami plots was corresponded to crystallinity in a wide range between 5% and 95%. Similar Avrami plots of Xt and the corresponded linear plots of PBS/BF (95/5) was obtained and shown in Supporting

Polymers 2017, 9, 351 8 of 14

Figure6a illustrates the Avrami plots of relative crystallinity ( Xt) versus crystallization time (t) obtained by exotherm data for the isothermal crystallization at various temperatures (PBS/MBF (95/5) as representative examples). The curves of PBS/MBF (95/5) composites showed S-shaped, which was related to nucleation and the growth process. It was also found that the time of isothermal crystallization completion was strongly dependent on Tc. As illustrated in Figure6b, a linearity in the Avrami plots was corresponded to crystallinity in a wide range between 5% and 95%. Similar Avrami Polymers 2017, 9, 351 8 of 14 plots of Xt and the corresponded linear plots of PBS/BF (95/5) was obtained and shown in Supporting Information (Figure S2). The Avrami parameters n and k can be obtained from the slope and the Information (Figure S2). The Avrami parameters n and k can be obtained from the slope and the intercept of Avrami plots and are summarized in Table1. intercept of Avrami plots and are summarized in Table 1. (a) (b) 2 100 a b c d 1 a b c d 80 0 60 o crystallinity(%) a) 72 C -1 o o 40 b) 74 C ln[-ln(1-Xt)] a) 72 C o o -2 b) 74 C c) 76 C o 20 c) 76 C d) 78oC d) 78oC Relative degree -3 0 0369121518 -10123 Time(min) lnt

Figure 6. (a) Relative degree of crystallinity with time and (b) the Avrami plots of ln[−ln(1 − X )] Figure 6. (a) Relative degree of crystallinity with time and (b) the Avrami plots of ln[−ln(1 − Xt)] versust versuslnt for isothermal lnt for isothermal crystallization crystallization of PBS/MBF of PBS/MBF (95/5) at (95/5) different at different crystallized crystallized temperatures. temperatures.

As shown in Table1 1,, thethe averageaverage n values of PBS and its composites ranged from 2.16 to 2.59. For pure PBS with high crystallizability, thethe nn values were from 2.40 to 2.58. These values were mainly attributed to a self-nucleation and the homogeneoushomogeneous nucleation mechanisms of PBS. After blending with reinforcedreinforced fibers,fibers, heterogeneousheterogeneous nucleationnucleation tooktook placeplace inin aa predominantpredominant wayway forfor PBS/MBFPBS/MBF composites. For For untreated untreated and and silane-treated fiber-reinforced fiber-reinforced composites, the average n values of PBS/BF (95/5) (95/5) and and PBS/MBF PBS/MBF (95/5) (95/5) composites composites were were 2.26 2.26 and and 2.30, 2.30, respectively. respectively. As As shown shown in POM images (Figure(Figure3), 3), there there were were two differenttwo different crystalline crystalline structures structures observed inobserved the PBS/MBF in the composites, PBS/MBF includingcomposites, spherulites including and spherulites transcrystalline and transcrystalli layers. Althoughne layers. TC structure Although was TC observed structure in was the PBS/MBFobserved composites,in the PBS/MBF transcrystallization composites, transcrystallization was essentially identicalwas essentially with spherulites identical with since spherulites these two since crystalline these structurestwo crystalline consist structures of the same consist lamellae of the and same the lamell sameae linear and growth the same rate linear [25,34 growth,35]. The rate visual [25,34,35]. difference The betweenvisual difference them is that between the spherulites them is that began the tospherulites grow radially began from to grow the nuclei, radially while from the the transcrystallines nuclei, while grewthe transcrystallines perpendicular togrew the surfaceperpendicular of silane-treated to the su fibers.rface of Thus, silane-treated the slight variationfibers. Thus, of the then values slight indicatedvariation of that the the n values incorporation indicated of untreatedthat the incorporation or silane-treated of untreated fibers into or PBSsilane-treated bulk did not fibers greatly into alterPBS thebulk growth did not dimensionality greatly alter the during growth the isothermaldimensionality crystallization during the process. isothermal This crystallization might also explain process. the phenomenonThis might also that explain spherulitic the phenomenon and TC structure that co-existedspherulitic in and higher TC structure MBF loading co-existed composites in higher (observed MBF inloading Figure composites3). At a given (observed crystallization in Figure temperature, 3). At a given the crystallizationk values of composites temperature, were thehigher k values than of thatcomposites of pure were PBS, higher which than implies that that of pure the heterogeneous PBS, which implies nucleation that the was heterogeneous accelerated bynucleation the addition was ofaccelerated fibers. As by the the amount addition of of basalt fibers. fiber As the loading amount increases, of basalt it isfiber also loading found increases, that the k itvalues is also further found increase,that the k which values would further accelerate increase, the which completion would ofaccelerate crystallization. the completion As a result, of incrystallization. combination withAs a POMresult, images, in combination it can be with concluded POM images, that the it incorporation can be concluded of basalt that fibers the incorporation contributedto of an basalt improved fibers heterogeneouscontributed to nucleationan improved ability, heterogeneous and the silane-treated nucleation basaltability, fibers and providedthe silane-treated abundant basalt nucleation fibers sitesprovided to induce abundant transcrystallinity nucleation sites in PBS. to induce However, transcrystallinity it is expected in that PBS. the However, formation it of is spherulitesexpected that or transcrystallinitythe formation of spherulites may not influence or transcrystallinity the growth rate may of not PBS influence crystals. the growth rate of PBS crystals. The half time of crystallization t1/2, usually defined as the time required to achieve 50% of the final crystallization kinetics, was introduced to evaluate the crystallization rate of crystalline or semi- crystalline polymers. The t1/2 values can be obtained as the following equation: ln2 / = (3) / where n and k can be achieved according to Equation (3).

Polymers 2017, 9, 351 9 of 14

The half time of crystallization t1/2, usually deﬁned as the time required to achieve 50% of the ﬁnal crystallization kinetics, was introduced to evaluate the crystallization rate of crystalline or semi-crystalline polymers. The t1/2 values can be obtained as the following equation:

ln 2 1/n t = (3) 1/2 k where n and k can be achieved according to Equation (3).

Table 1. Avrami parameters of isothermal crystallization kinetics of PBS, PBS/BF, and PBS/MBF.

◦ −1 −n Sample Tc ( C) t1/2 (min) 1/t1/2 (min ) k·(min ) n Average (n) 72 2.68 0.373 0.054 2.58 74 4.50 0.222 0.017 2.47 PBS 2.47 76 7.30 0.137 0.006 2.43 78 11.4 0.088 0.002 2.40 72 2.28 0.439 0.080 2.67 74 3.42 0.292 0.031 2.55 PBS/MBF (98/2) 2.59 76 4.88 0.205 0.013 2.55 78 7.53 0.133 0.004 2.58 72 1.63 0.613 0.232 2.34 74 2.32 0.413 0.102 2.33 PBS/BF (95/5) 2.26 76 3.47 0.288 0.046 2.20 78 5.33 0.188 0.020 2.17 72 1.58 0.633 0.245 2.36 74 2.22 0.45 0.115 2.30 PBS/MBF (95/5) 2.30 76 3.25 0.308 0.049 2.27 78 4.88 0.205 0.020 2.25 72 1.27 0.787 0.420 2.36 74 1.75 0.571 0.201 2.28 PBS/MBF (90/10) 2.26 76 2.57 0.389 0.088 2.23 78 3.82 0.262 0.039 2.16 72 1.18 0.847 0.499 2.26 74 1.52 0.658 0.293 2.16 PBS/MBF (85/15) 2.16 76 2.18 0.459 0.137 2.12 78 3.34 0.299 0.058 2.09

The obtained t1/2 values for PBS and its composites are listed in Table1. As expected, with increasing crystallization temperature Tc, the t1/2 value of all the samples correspondingly increased, suggesting a slower crystallization rate. In the presence of untreated and silane-treated basalt fiber, the t1/2 values became shorter, exhibiting a heterogeneous nucleating effect. Moreover, by increasing the content of silane-treated fiber, the values of t1/2 further decreased, indicating an accelerated crystallization rate. ◦ For example, at a given Tc of 78 C, the t1/2 values of neat PBS is 11.4 min, whereas PBS/BF (95/5) and PBS/MBF (98/2, 95/5, 90/10, and 85/15) completed the crystallization within 5.33, 7.53, 4.88, 3.82, and 3.34 min, respectively. Figure7 illustrates the variation of 1/ t1/2 with Tc for all of the samples. Conveniently, the reciprocal of t1/2 represented the crystallization rate. Both PBS and its composites exhibited an increase of 1/t1/2 values with the reduction of Tc due to a greater super-cooling at lower Tc, which accelerated the crystallization rate. These enhanced crystallization rates with decreasing Tc further confirm that the initial nucleation dominated the isothermal crystallization rate of the samples. On the other hand, the 1/t1/2 value of silane-treated fiber-reinforced PBS composites was greater than that of untreated samples. In particular, the 1/t1/2 values further prolonged with the increase content of silane-treated basalt fiber. During the crystallization process, the presence of pristine and silane-treated basalt fiber served as nucleating agent for PBS. Furthermore, the silane-treated one presented a better heterogeneous nucleation effect. Polymers 2017, 9, 351 10 of 14 Polymers 2017, 9, 351 10 of 14 Polymers 2017, 9, 351 10 of 14 1.0 1.0 PBS PBS PBS/MBF(98/2) 0.8 PBS/MBF(98/2) PBS/BF(95/5)

) 0.8 PBS/BF(95/5) PBS/MBF(95/5) -1

) PBS/MBF(95/5) PBS/MBF(90/10) -1 0.6 PBS/MBF(90/10) PBS/MBF(85/15)

(min 0.6 PBS/MBF(85/15) (min 1/2 1/2

1/t 0.4

0.2 0.2

0.00.0 7272 74 74 76 76 78 78 TT (°C)(°C) cc

Figure 7. Variation of 1/t with T for PBS, PBS/BF and PBS/MBF composites. FigureFigure 7. 7. Variation Variation ofof 11/t1/2/t1/21/2 with Tcc forfor PBS,PBS, PBS/BF PBS/BF and and PBS/MBF PBS/MBF composites. composites.

3.4.3.4. Tensile Tensile PropertiesProperties Properties ofof of PBS/MBFPBS/MBF PBS/MBF Biocomposites BiocompositesBiocomposites InInIn order order toto to explore explore the the relationship relationship betweenbetween the the transcrystalline transcrystalline structure structurestructure and andand macro macro mechanical mechanical performance,performance,performance, the the tensile tensile propertyproperty ofof compositescomposites in in accordance accordanceaccordance with withwith ASTM ASTMASTM standards standardsstandards was waswas tested testedtested (Figure(Figure(Figure8 8a).a). 8a). The The typical typicaltypical stress-strain stress-strain curvescurves curves of of neatneat neat PBSPBS PBS and and and untreated untreated untreated and and silane-treated and silane-treated silane-treated basalt basalt fiber basalt fiber fiberreinforcedreinforced reinforced composites composites composites areare shown areshown shown inin Figure in Figure 8b8b 8 andband and TableTable Table 2.2.2 .PBS PBS exhibited exhibitedexhibited a aayielding yieldingyielding behavior behavior behavior followedfollowedfollowed byby by aa coldacold cold drawing, drawing, whichwhich which isisis typicaltypical forfor ductile ductile polymers. polymers. On On the the other other hand, hand, the the stress-strain stress-strain behaviorbehavior of of basalt basalt fiber-reinforcedfiber-reinforced fiber-reinforced composites compositescomposites alteredaltered considerably considerablyconsiderably from fromfrom the thethe neat neatneat PBS. PBS. The The elongation elongation at break decreased dramatically with almost linear tensile behavior. As can be inferred from Table 2, at break decreased dramatically with almost linear tensile behavior. As can be inferred from Table Table 22,, untreated BF and silane-treated BF with same mass loading showed some difference in enhancement untreated BFBF and silane-treated BFBF with same massmass loading showed some difference in enhancement in tensile properties. For the PBS matrix, the tensile strength and modulus was 23.2 and 140.6 MPa, inin tensiletensile properties.properties. For the PBS matrix, the tensiletensile strength and modulus was 23.2 and 140.6 MPa, respectively. For composites reinforced with pristine basalt fiber (PBS/BF (95/5)), the tensile strength respectively.respectively. ForFor compositescomposites reinforcedreinforced with with pristine pristine basalt basalt fiber fiber (PBS/BF (PBS/BF (95/5)),(95/5)), the tensile strength and modulus was enhanced by 7.3 and 126.8 MPa, respectively, which was about 31.5% and 77.7% and modulus was enhanced by 7.3 and 126.8 MPa, respectively, which was about 31.5% and 77.7% andhigher modulus than thatwas of enhanced PBS matrix. by However,7.3 and 126.8 for the MPa, PBS/MBF respectively, (95/5) biocomposite which was s,about tensile 31.5% strength and and 77.7% highermodulus thanthan were thatthat ofof34.5 PBSPBS and matrix.matrix. 290.0 However,MPa,However, which forfor significan thethe PBS/MBFPBS/MBFtly increased (95/5)(95/5) biocompositeby biocomposites, 48.7% and s,144% tensile in contraststrength to and modulusneat PBS, werewere respectively. 34.534.5 and and 290.0 290.0At a MPa, higherMPa, which whichsilane-treated significantly significan BFtly increasedloading, increased the by tensile 48.7%by 48.7% andstrength and 144% 144% and in contrast modulusin contrast to of neat to PBS,neatPBS/MBF respectively.PBS, respectively. composites At a higherwere At a furtherhigher silane-treated boosted.silane-treated BF For loading, ex ample,BF loading, the as tensile the theMBF strength tensile content andstrength increased modulus and to of modulus15%, PBS/MBF the of compositesPBS/MBFtensile strength composites were furtherand weremodulus boosted. further were boosted. For increased example, For exby asample, 93.1% the MBF asand the content 281.3%, MBF content increased compared increased to with 15%, neatto the 15%, tensilePBS the strengthtensileaccordingly. strength and modulus Accordingly, and modulus were surface increased were modification increased by 93.1% coul andbyd 93.1%contribute 281.3%, and compared to 281.3%, an improved withcompared PBS/MBF neat PBS with interfacial accordingly. neat PBS Accordingly,accordingly.compatibility, surfaceAccordingly, preventing modification surfacemicrovoids couldmodification and contribute fiber-PBS coul to ddebonding an contribute improved in to the PBS/MBF an interface improved interfacial region. PBS/MBF compatibility, interfacial preventingcompatibility, microvoids preventing and microvoids fiber-PBS debonding and fiber-PBS in the debonding interface in region. the interface region.

Figure 8. (a) Standard tensile samples; (b) stress-strain curve of PBS, PBS/BF, and PBS/MBF.

FigureFigure 8.8. ((aa)) StandardStandard tensiletensile samples;samples; ((bb)) stress-strainstress-strain curvecurve of of PBS, PBS, PBS/BF, PBS/BF, andand PBS/MBF.PBS/MBF.

Polymers 2017, 9, 351 11 of 14

FigurePolymers9 showed 2017, 9, 351 the tensile fracture topography of PBS, PBS/BF, and PBS/MBF composites11 of 14 at a highFigure magnification. 9 showed the For tensile PBS/BF fracture (95/5) topography biocomposites, of PBS, PBS/BF, visible and interfacial PBS/MBF composites gaps were at founda betweenhigh themagnification. fiber and PBSFor PBS/BF matrix, (95/5) and biocomposites, fiber-PBS debonding visible interfacial in the interphase gaps were found region between can be the clearly observedfiber (Figureand PBS9 c),matrix, which and suggests fiber-PBS poor debonding interfacial in adhesionthe interphase between region the can untreated be clearly fibers observed and PBS. For PBS/MBF(Figure 9c), composites which suggests (Figure poor9b,d–f), interfacial the polymers adhesion arebetween coated the on untreated the silane-treated fibers and fiberPBS. surface,For and thePBS/MBF voids betweencomposites PBS (Figure matrix 9b,d–f), and the MBF polymers were significantly are coated on reduced, the silane-treated suggesting fiber strong surface, interfacial and adhesionthe voids compared between to thePBS untreatedmatrix and fiber. MBF were significantly reduced, suggesting strong interfacial adhesion compared to the untreated fiber.

FigureFigure 9. Scanning 9. Scanning electron electron microscopy microscopy (SEM) (SEM) micrographsmicrographs of of tensile tensile fractured fractured surfaces surfaces of (a of) PBS, (a) PBS, (b) PBS/MBF(b) PBS/MBF (98/2), (98/2), (c) PBS/BF(c) PBS/BF (95/5), (95/5), ( d(d)) PBS/MBF PBS/MBF (95/5), (e ()e )PBS/MBF PBS/MBF (90/10), (90/10), and and (f) PBS/MBF (f) PBS/MBF (85/15).(85/15).

In order to a obtain deeper understanding on the effect of interfacial crystallization on tensile Inresults order and to ainterfacial obtain deeper strength, understanding a simulation onof thematerial effect ofproperties interfacial was crystallization performed using on tensilea resultsmathematical and interfacial Kelly-Tyson strength, model a simulation derived of from material expe propertiesrimental data was of performed PBS/MBF biocomposites, using a mathematical as Kelly-Tysongiven by modelEquation derived (4) [36,37]. from experimental data of PBS/MBF biocomposites, as given by

Equation (4) [36,37]. =⁄ + (4) σc = τi(L/d)VfCo + σmVm (4) where σm and σc respectively represent the tensile strength of matrix and its composites, τi represents wheretheσm interfacialand σc respectively shear strength represent (ISS), L theand tensiled are the strength fiber length of matrix and diameter, and its composites, Co is the orientationτi represents parameter, Vm and Vf, are the fiber and matrix volume fraction, respectively. Especially, Vf is the interfacial shear strength (ISS), L and d are the ﬁber length and diameter, Co is the orientation calculated by Equation (5) [38]. parameter, Vm and Vf, are the ﬁber and matrix volume fraction, respectively. Especially, Vf is calculated 1− by Equation (5) [38]. =1+ (5) ρf 1 − Wf Vf = 1 + (5) ρm Wf Polymers 2017, 9, 351 12 of 14

where ρf and ρm are the density of polymer matrix and basalt fiber, respectively. Wf is the fiber weight fraction in the composite specimen. Following the Kelly-Tyson model, τi was obtained throughout Equations (4) and (5) and given in Table2. The average fiber diameter ( d) and the fiber length (L) of composites were obtained from optical microscopy. For short fiber reinforced composites, the empirical value of Co for random orientation of fiber in 3D is usually 0.2 [39]. As listed in Table2, the τi of PBS/BF (95/5) biocomposites was 50.2 MPa, while that of PBS/MBF (95/5) was 76.1 MPa, indicating that the silane treatment would contribute to an improved polymer/fiber interfacial bonding. This is also supported by the observation of interfacial crystallization from POM images as shown in Figures3 and4. The higher computed τi of PBS/MBF composites suggested better interfacial bonding between the MBF and PBS matrix. However, the τi of PBS/MBF composites did not increased with the increase content of MBF loading. The highest value of τi of PBS/MBF composites were obtained for PBS/MBF (98/2) biocomposites at a value of 100.1 MPa, which might be due to the more effective formation of interfacial crystallization of PBS to MBF of the composites at lower content of fiber loading. Therefore, the enhanced interfacial adhesion may interpret the improvement of tensile strength of PBS/MBF in contrast with that of PBS/BF composites from the perspective of interfacial crystalline structure.

Table 2. IFSS of PBS/BF and PBS/MBF composites.

σ τ Sample m σ (MPa) E (MPa) W (%) V (%) L (µm) i (MPa) ct f f (MPa) PBS/MBF (98/2) 23.2 29.8 ± 1.1 163.2 ± 5.6 2 1 317.3 100.1 PBS/BF (95/5) 23.2 30.5 ± 0.5 267.4 ± 32.5 5 2.3 315.5 50.2 PBS/MBF (95/5) 23.2 34.5 ± 0.3 290.0 ± 19.6 5 2.3 314.2 76.1 PBS/MBF (90/10) 23.2 39.6 ± 0.7 373.9 ± 22.3 10 4.8 307.3 55.2 PBS/MBF (85/15) 23.2 45.8 ± 1.0 442.3 ± 16.8 15 7.3 303.2 51.0

4. Conclusions Silane treatment was adopted to modify the surface of basalt fibers and improve the interfacial adhesion between PBS matrix and basalt fibers in PBS/MBF biocomposites. It was found that the transcrystalline structure was successfully induced and grew perpendicular to the silane-treated fiber axis, while no TC structure was observed in neat PBS or PBS/BF composites. The overall isothermal crystallization kinetics of PBS with varying silane-treated BF content at different crystallization temperatures was investigated by the Avrami equation. It is observed that the crystallization half-time (1/t1/2) values of PBS/MBF composites was shorter than that of PBS and its composites reinforced by pristine BF. The silane-treated basalt fiber acted as an efficient nucleating agent and provided abundant nucleating sites, accelerating the completion the crystallization process of PBS. Tensile results showed that the incorporation of silane-treated BF lead to an increase in tensile strength and modulus. The IFSS analysis verified that the interfacial crystalline structure improved PBS/BF interfacial bonding, which was also demonstrated by SEM images. It is concluded that silane treatment of BF may provide an effective method for preparing high-performance and biodegradable composites.

Supplementary Materials: The following are available online at www.mdpi.com/2073-4360/9/8/351/s1, Figure S1: The growth of polymeric matrix (PBS) spherulites in the presence of pristine basalt fibers crystallized at 89 ◦C at different times; Figure S2: (a) Relative degree of crystallinity with time and (b) the Avrami plots of ln[−ln(1 − Xt)] versus lnt for isothermal crystallization of PBS/BF (95/5) at different crystallized temperatures. Acknowledgments: The work was financially supported by the National Natural Science Foundation of China (Nos. 31500767), the China Postdoctoral Science Foundation (2015M571300), and the Fundamental Research Funds for the Central Universities (DUT17RC(4)57). Author Contributions: Wei Zhiyong and Sang Lin conceived and designed the experiments; Liang Qiushi and Zhao Mingyuan performed the experiments; Zhao Mingyuan and Liang Qiushi analyzed the data; Wei Zhiyong contributed reagents/materials/analysis tools; Sang Lin wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. Polymers 2017, 9, 351 13 of 14

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