And Polybutylene Succinate Adipate (PBSA) Copolymer During Degradation in Various Environmental Conditions

Article Molecular and Supramolecular Changes in Polybutylene Succinate (PBS) and Polybutylene Succinate Adipate (PBSA) Copolymer during Degradation in Various Environmental Conditions

Michał Puchalski 1,*, Grzegorz Szparaga 1, Tadeusz Biela 2, Agnieszka Gutowska 3,4, Sławomir Sztajnowski 1 and Izabella Kruci ´nska 1

1 Faculty of Material Technologies and Textile Design, Department of Material and Commodity Sciences and Textile Metrology, Centre of Advanced Technologies of Human-Friendly Textiles “Pro Humano Tex”, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland; [email protected] (G.S.); [email protected] (S.S.); [email protected] (I.K.) 2 Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-301 Lodz, Poland; [email protected] 3 Institute of Security Technologies “MORATEX”, Sklodowskiej-Curie 3, 90-505 Lodz, Poland; [email protected] 4 Department of Synthetic Fibers, Institute of Biopolymers and Chemical Fibres, Skłodowskiej-Curie 19/27, 90-570 Lodz, Poland * Correspondence: [email protected]; Tel.: +48-42-631-33-22

Received: 26 January 2018; Accepted: 27 February 2018; Published: 1 March 2018

Abstract: In this paper, the inﬂuence of the various degradation conditions, on the molecular and supramolecular structure of polybutylene succinate (PBS) and polybutylene succinate adipate (PBSA) copolymer during degradation is described. The experiment was carried out by the use of injection molded samples and normalized conditions of biodegradation in soil, composting and artiﬁcial weathering. Materials were studied by size-exclusion chromatography (SEC) coupled with multiangle laser light scattering (MALLS) detection and wide-angle X-ray diffraction (WAXD). Additionally, the physical and mechanical properties of the samples were determined. The performed experiments clearly show difference impacts of the selected degradation conditions on the macroscopic, supramolecular and molecular parameters of the studied aliphatic polyesters. The structural changes in PBS and PBSA explain the observed changes in the physical and mechanical properties of the obtained injection molded samples.

Keywords: PBS; PBSA; WAXD; SEC-MALLS; degradation; composting; artiﬁcial weathering

1. Introduction In recent years, pro-ecological trends in waste reduction have led to the substitution of durable polymers with biodegradable polymers. For this, thermoplastic aliphatic polyesters are promising alternatives [1,2]. In this group of polymers, polybutylene succinate (PBS) is an interesting material from an application standpoint, as its mechanical properties are similar to those of popular polymers, such as polypropylene (PP) [3]. Polybutylene succinate (PBS) is a biodegradable aliphatic polyester produced by the polycondensation of succinic acid (SA) and 1,4-butanediol (BD) [4]. The synthesis and characterization of PBS and polybutylene adipate (PBA), which is synthesized from 1,4-butanediol (BD) and adipate acid (AA), are extensively described in the literature using mathematical models [5,6]. Due to the high crystallinity and good thermal properties of the homopolymers, a copolymer of PBS and PBA has been used in

Polymers 2018, 10, 251; doi:10.3390/polym10030251 www.mdpi.com/journal/polymers Polymers 2018, 10, 251 2 of 12 selected applications, such as packaging [7]. The physical properties and crystalline structure of the polybutylene succinate adipate (PBSA) copolymer strongly depend on the composition of the polymer compounds. Moreover, the PBSA copolymer is characterized by a higher degradability by enzymatic processes compared with other materials due to its lower crystallinity [8]. For many years, PBS and PBSA have been produced from petrochemical sources by Showa Highpolymer (Shanghai, China). The polymer, which has the trade name Bionolle, is characterized by its similar processability to that of conventional resins, such as polyethylene. Bionolle is one of the most suitable materials for processing into films, which can then be utilized for agricultural purposes, shopping bags, compost bags, and so on. Showa Highpolymer has succeeded in producing a compound of Bionolle and starch that not only has similar properties to homogeneous Bionolle but is also environmentally friendly [9–11]. The lack of renewability and the rising price of fossil resources have limited the use of petrochemical succinic acid for a wide range of applications. Consequently, the natural next step was to develop a synthetic method for bio-succinic acid from a renewable source, such as biomass. The yeast- or bacteria-based production of succinic acid and the possibility of sourcing 1,4-butanediol from ecological methods make PBS and its copolymers attractive biodegradable polymers that are completely produced from renewable resources [12–14]. Currently, research on this topic is related to the search for new applications of PBS, for example, the development of novel materials for ecological agriculture purposes. Developed materials, such as mulching nonwovens and pots produced from nonwovens, are interesting alternatives to polypropylene products [15,16]. Another area of work is the use of polybutylene succinate as an additive to plasticize other biodegradable polymers, such as polylactide (PLA) [17–19]. Research is therefore concerned with the search for new applications for this aliphatic polyester [20–22]. On the other hand, research has focused on the analysis of the degradation of PBS, PBA and their copolymers by the examination of new degradation conditions, enzymes and microorganisms [8,23–25]. This topic is still developing and continues to expand the understanding of this polymer and other aliphatic polyesters. The impacts of the degradation process on weight loss, the decrease in the molar mass and changes in mechanical properties have been evaluated [26,27]. The kinetics of the degradation process have also been analyzed [28]. However, there are few results for the assessment of changes in the supramolecular structure during the PBS degradation process [29]. For other aliphatic polyesters (e.g., PLA), such changes have been observed and were responsible, inter alia, for a decrease in the mechanical properties [30]. The influences of properties and structures of aliphatic polyesters and their copolymers, such as molar mass, thermal properties, degree of crystallinity or the ratio between ester bonds on their ability to degrade, were also analyzed [31–34]. In this study, the degradation of a commercially available PBS and PBSA copolymer in three various regimes—biodegradation in compost, biodegradation in soil and artificial weathering—was performed. The experiment was carried out under laboratory conditions according to the obligatory standard. For complete analysis, the change in the studied samples during degradation was analyzed on the macroscopic, supramolecular and molecular scales. The estimation of mass loss and change in the mechanical properties was carried out according to the obligatory standards, while supramolecular and molecular parameters were investigated by using wide-angle X-ray diffraction (WAXD) and size-exclusion chromatography (SEC) coupled with multiangle laser light scattering (MALLS) detection.

2. Materials and Methods

2.1. Materials In the experiments, the commercially available polybutylene succinate (PBS) Bionolle 1020 MD and the polybutylene succinate adipate (PBSA) copolymer Bionolle 3020 MD were purchased from Showa Denko K.K. (Tokyo, Japan). For the tensile test, the samples were formed into dog-bone shaped Polymers 2018, 10, 251 3 of 12 specimens by the use of an injection molding machine (Allrounder 420C, Arburg, Loßburg, Germany) according to the ISO-527-2-1A standard. The polymer materials were formed at 180 ◦C (PBS, 1020 MD) and 170 ◦C (PBSA, 3020 MD).

2.2. Degradation Environment

2.2.1. Laboratory Biodegradation The laboratory biodegradation of the PBS and PBSA samples was conducted under controlled conditions in accordance with the International Standard, PN-EN ISO 20200:2016, and European Standards, PN-EN 14806:2010 and PN-EN 14045:2005. The experiment was carried out using common commercially available garden soil with a pH of 6.0–6.5 and 1.8 × 107 cfu/g microorganisms and compost from an industrial compost prism (Municipal Services Company of the city of Łód´z,Łód´z, Poland) with a pH of 7 and 3.2 × 107 cfu/g microorganisms [15,35]. The studied samples were biodegraded in the garden soil at a temperature of 30 ± 2 ◦C, and the medium had a moisture content of 55.6%, while the samples were biodegraded in compost at a temperature of 58 ± 2 ◦C, and the medium had a moisture content of 53.1%, which was in line with the adopted standards. The biodegradation processes were controlled to have deﬁned time intervals (1, 4, 8, 12, 16, 20 and 24 weeks). During the experiment, moisture was replenished to the initial value with water. After a deﬁned incubation time, the samples were dried to a constant weight, and the mass loss was estimated. Each sample was tested 3 times. The resulting variation in the results of the biodegradation tests was below 10%.

2.2.2. Laboratory Artificial Weathering The laboratory artificial weathering of the studied PBS and PBSA samples was carried out in a QUV chamber, in accordance with the standard, PN-EN ISO 4892-3, based on the Technical Report TR 010 ed. May 2004 “Exposure procedure for artificial weathering”. The weathering process was performed in cycles, irradiating the samples with a light intensity of 0.76 W/m2 by UVA-340 nm radiation under the following conditions:

• 8 h of exposure—chamber temperature (ChT) 50 ◦C, relative humidity (RH) 40%; • 4 h of artiﬁcial rainfall with demineralized water and no exposure—ChT = 20 ◦C.

The samples were weathered for deﬁned time intervals—45, 90, 180, 360, 540, 720, 900, 1080, 1260, 1440 and 1800 h—where the 720 h interval corresponded to the natural exposure of a sample over 1 year in a moderate climate (e.g., in Poland).

2.3. Mechanical Properties The mechanical parameters of the PBS and PBSA samples that did not fall apart during degradation were measured using an Instron 5511 mechanical testing machine (Instron, Canton, MA, USA). The tests were carried out according to the PN-EN ISO 527-2:2012 standard.

2.4. SEC-MALLS Method

The molar mass (Mn) and dispersity (Đ) of the studied PBS and PBSA materials were analyzed by size-exclusion chromatography (SEC) coupled with multiangle laser light scattering (MALLS) detection. The SEC−MALLS instrument was composed of an Agilent 1100 isocratic pump, an auto-sampler, a degasser, a thermostatic box for columns, a MALLS DAWN HELEOS-II photometer (Wyatt Technology Corporation, Santa Barbara, CA, USA) and an Optilab T-rEX differential refractometer. The ASTRA 4.90.07 software package (Wyatt Technology Corporation) was used for data collection and processing. Two PLGel 5 µm MIXD-C columns were used for separation. The samples were injected as a methylene chloride solution. The volume of the injection loop was 100 µm3. Methylene chloride was used as the mobile phase at a ﬂow rate of 0.8 cm3·min−1. The calibration of the DAWN HELEOS-II photometer was performed using p.a. grade toluene, and normalization was Polymers 2018, 10, 251 4 of 12 Polymers 2018, 10, x FOR PEER REVIEW 4 of 12 performedwas performed using ausing polystyrene a polystyrene standard standard (Mn = 30,000 (Mn g/mol). = 30,000 The g/mol). measurements The measurements were conducted were at roomconducted temperature. at room temperature.

2.5.2.5. WAXSWAXS MethodMethod TheThe changeschanges inin thethe supramolecularsupramolecular structuresstructures ofof thethe PBSPBS andand PBSAPBSA samplessamples duringduring degradationdegradation werewere investigatedinvestigated byby wide-anglewide-angle X-rayX-ray diffractiondiffraction scatteringscattering (WAXS) usingusing anan X’PertX’Pert PROPRO diffractometerdiffractometer (CuK(CuKαα source,source, λλ == 0.1540.154 nm)nm) from PANalyticalPANalytical (Almelo,(Almelo, TheThe Netherlands).Netherlands). TheThe X-rayX-ray diffractogramsdiffractograms werewere recorded recorded in in the the 2θ 2rangeθ range of 5of◦ to5° 60to◦ 60°with with a step a step of 0.05 of ◦0.05°.. The The obtained obtained WAXS WAXS data weredata analyzedwere analyzed numerically numerically using the using WAXSFIT the WAXS softwareFIT (2.0,software University (2.0, ofUniversity Bielsko Biała, of Bielsko Bielsko-Biała, Biała, Poland)Bielsko-Bia [36].ła, Poland) [36].

3.3. ResultsResults andand DiscussionDiscussion 3.1. Analysis of the Physical Properties of the Studied Materials after Degradation 3.1. Analysis of the Physical Properties of the Studied Materials after Degradation The physical changes in the structures of the studied materials were directly observed and The physical changes in the structures of the studied materials were directly observed and recorded photographically. In Figure1, a collection of selected photographs of the degradation recorded photographically. In Figure 1, a collection of selected photographs of the degradation processes occurring in various conditions is presented. According to these photographs, the most processes occurring in various conditions is presented. According to these photographs, the most degrading medium was compost. During PBSA biodegradation in compost, the tested sample began degrading medium was compost. During PBSA biodegradation in compost, the tested sample began to fragment after 4 weeks, and PBS began to fragment after 6 weeks. The other degradation conditions to fragment after 4 weeks, and PBS began to fragment after 6 weeks. The other degradation applied in the investigation did not result in fragmentation of the studied materials. Matting of conditions applied in the investigation did not result in fragmentation of the studied materials. the samples’ surfaces and additional color changes are the only visible changes resulting from Matting of the samples’ surfaces and additional color changes are the only visible changes resulting artiﬁcial weathering. from artificial weathering.

Figure 1. Photographs of the investigated samples before and after degradation. Figure 1. Photographs of the investigated samples before and after degradation. The most often analyzed macroscopic factor in material degradation assessments is mass loss. In FigureThe most 2, the often mass analyzed changes macroscopic of the studied factor PBS in and material PBSA degradationsamples as a assessments function of isdegradation mass loss. Intime Figure are shown.2, the mass The most changes visible of the mass studied loss was PBS measured and PBSA for samples the samples as a functionbiodegraded of degradation in compost. time(Figure are 2a), shown. while The the most weakest visible initiator mass lossof mass was loss measured was artificial for the samplesweathering, biodegraded where the in maximum compost. (Figuremass loss2a), was while only the around weakest 1% initiator for both of studied mass loss ma wasterials artiﬁcial (Figure weathering, 2c). The insignificant where the maximumchanges in massthe sample loss was mass only during around aging 1% for under both simulated studied materials atmospheric (Figure conditions2c). The insigniﬁcant seems to be changes a result inof thethe sampleabsence mass of its during direct aging physical under contact simulated with atmospheric the degrading conditions medium. seems In the to be soil a result or in ofcompost, the absence the ofdegraded its direct products physical probably contact withdiffuse the from degrading the surface medium. layer into In the the soil environment, or in compost, which the is degraded observed productsas a decrease probably in mass. diffuse Furthermore, from the surface in compost, layer into a the visible environment, strong mass which loss is observedalso results as a from decrease the reaction with enzymes that support the erosion of the surface of the studied material.

Polymers 2018, 10, x FOR PEER REVIEW 5 of 12

Polymers 2018, 10, 251 5 of 12 Analysis of the difference between both studied Bionolle polymers clearly shows a higher mass loss from the PBSA material than the PBS material over the same time period. It is worth noting that inthe mass. PBS polymer Furthermore, lost a significant in compost, amount a visible of mass strong only mass during loss biodegradation also results from in compost, the reaction and other with enzymesinitiators thatdid not support cause the a decrease erosion ofin theits mass. surface of the studied material.

Figure 2.2. The mass changes of the studied samples duringduring degradation in selected environments: ((a)) biodegradationbiodegradation inin compost,compost, ((bb)) biodegradationbiodegradation inin soilsoil andand ((cc)) artiﬁcialartificial weathering.weathering.

AnalysisThe next ofstep the in difference the macroscopic between analysis both studied of physical Bionolle changes polymers of the clearly studied shows samples a higher was mass the lossinvestigation from the PBSAof their material mechanical than properties. the PBS material In Figure over 3, the the same stress time and period.elongation It is at worth break-point noting thatas a thefunction PBS polymer of degradation lost a significant time is shown. amount As of it massis seen, only there during is a biodegradationlack of information in compost, about changes and other in initiatorsthe mechanical did not properties cause a decrease of the studied in its mass. samples during biodegradation in compost. Samples that wereThe composted next step for in more the macroscopic than 1 or 4 weeks analysis were of physicalfragmented changes into smaller of the studiedpieces, which samples prevented was the investigationmechanical testing of their (Figure mechanical 1). The properties. stress and In Figure elongation3, the stressat break-point and elongation of PBS at break-pointafter 4 weeks as a(maximum function of time degradation after which time the is sample shown. is As not it frag is seen,mented) there decreased is a lack of from information 34.8 MPa about to 12.7 changes MPa and in the321.8% mechanical to 1.7% respectively, properties of while the studied for PBSA, samples after during1 week, biodegradation these values decreased in compost. from Samples 28.6 MPa that to were21.8 MPa composted and from for 757.9% more thanto 196.1%. 1 or 4 These weeks results were fragmented indicate the intostrong smaller degradation pieces, whichof both prevented polymers mechanicalin compost, testing which (Figure is probably1). The an stress effect and of elongation its composition at break-point being more of PBS complex after 4 weeks than (maximumsoil from a timemicrobiological after which (enzymatic) the sample ispoint not fragmented)of view. The decreasedchanges in from the mechanical 34.8 MPa to properties 12.7 MPa andof the 321.8% studied to 1.7%polymers respectively, as a function while forof PBSA,degradation after 1 week,time clearl thesey values show decreaseda large decrease from 28.6 in MPathe toelasticity 21.8 MPa of andthe frommaterials 757.9% during to 196.1%. the degradation These results process, indicate both the in strong the biodegradation degradation of bothprocess polymers in soil inand compost, in the whichirradiation is probably process an of effectartificial of its weathering, composition as beingpresented more in complex Figure 3. than Furthermore, soil froma as microbiological a result of the (enzymatic)artificial weathering point of view.process, The the changes strength in theof both mechanical studied properties polymers ofwas the significantly studied polymers reduced as a(Figure function 3b). of Considering degradation the time lack clearly of mass show loss a and large matting decrease of inthe the sample elasticity surface, of the it can materials be concluded during thethat degradation the structure process, of the polymers both in the changed biodegradation on a molecular process and in soila supramolecular and in the irradiation level, as processdiscussed of artificialin the next weathering, part of the aspaper. presented in Figure3. Furthermore, as a result of the artificial weathering process,Slightly the strengthdifferent of changes both studied in the polymersstress of the was polymers significantly were reduced observed (Figure in the3b). biodegradation Considering theprocess lack in of soil mass (Figure loss and 3a). matting For both of thePBS sampleand PBSA, surface, the itrates can of be change concluded were that lower the than structure during of thedegradation polymers under changed artificial on a weathering molecular and conditions. a supramolecular However, level,in the ascase discussed of a polymer in the containing next part an of theadipate paper. functional group, the strength of the tested samples decreased considerably with the time of degradationSlightly and different after changes24 weeks, in was the stressat a level of the of polymers4 MPa. Based were on observed the analysis in the of biodegradation the estimated processmacroscopic in soil factors, (Figure it3 a).can Forbe concluded both PBS andthat PBSA, the most the ratesdegrading of change environment were lower is compost. than during This degradationcould be expected, under because artificial in weathering the compost conditions. degradation However, process, in carried the case out of in a accordance polymer containing with the anISO adipate standard, functional the degradation group, the of strengthpolymers of occurs the tested at least samples for two decreased processes: considerably hydrolysis, withwhich the is timesupported of degradation by moisture and in after the 24medium weeks, [29], was and at a levelbiodegradation, of 4 MPa. which Based onis realized the analysis by natural of the estimatedenzymes and macroscopic microorganisms factors, it[37,38]. can be Less concluded intense that effects the mostof polymer degrading degradation environment were is compost.obtained Thisusing could soil, bewhich expected, is the becauseresult of in the the lower compost content degradation of microorganisms process, carried and enzymes out in accordance as well as with the

Polymers 2018, 10, 251 6 of 12

the ISO standard, the degradation of polymers occurs at least for two processes: hydrolysis, which is supported by moisture in the medium [29], and biodegradation, which is realized by natural enzymes Polymersand microorganisms2018, 10, x FOR PEER [37 REVIEW,38]. Less intense effects of polymer degradation were obtained using6 of 12 soil, which is the result of the lower content of microorganisms and enzymes as well as the realizing of the realizingprocess of at the lower process temperatures, at lower temperatures, in accordance in with accordance the ISO with standard the ISO used, standard compared used, to compared the case of to degradationthe case of indegradation compost. Notably, in compost. both ofNotably, applied degradationboth of applied conditions degradation showed conditions greater susceptibility showed greaterto degradation susceptibility of the to polymerdegradation containing of the polymer the adipate containing functional the group,adipate which functional is consistent group, which with the is currentconsistent state with of knowledgethe current [state34]. of knowledge [34].

Figure 3. The changes in the mechanical properties of the studied samples during degradation in Figure 3. The changes in the mechanical properties of the studied samples during degradation in selected environments: (a) biodegradation in soil and (b) artificial weathering. selected environments: (a) biodegradation in soil and (b) artiﬁcial weathering.

3.2. Changes in the Molar Mass and Dispersity of the Studied Materials during Degradation 3.2. Changes in the Molar Mass and Dispersity of the Studied Materials during Degradation The molecular parameters (Mn and Mw/Mn = Đ) of the studied polymers were analyzed using The molecular parameters (Mn and Mw/Mn = Đ) of the studied polymers were analyzed using the SEC-MALLS method. In Figure 4, the number average molar mass (Mn) and dispersity (Ð) [39] of the SEC-MALLS method. In Figure4, the number average molar mass ( M ) and dispersity (Ð)[39] of the studied samples as a function of degradation time under the selected conditionsn are presented. the studied samples as a function of degradation time under the selected conditions are presented. The most visible change in the molecular parameters of the studied polymers was observed for the The most visible change in the molecular parameters of the studied polymers was observed for the samples degraded by the artificial weathering process. Exposure of the sample to ultra violet (UV) samples degraded by the artificial weathering process. Exposure of the sample to ultra violet (UV) radiation, high relative humidity and heat resulted in a decrease in the number average molar radiation, high relative humidity and heat resulted in a decrease in the number average molar weight weight and an increase in the dispersity of both studied polymers (Figure 4c). During this process, and an increase in the dispersity of both studied polymers (Figure4c). During this process, the value the value of Mn decreased from 37.4 kg/mol and 37.8 kg/mol, to 12.1 kg/mol and 11.0 kg/mol, for PBS of M decreased from 37.4 kg/mol and 37.8 kg/mol, to 12.1 kg/mol and 11.0 kg/mol, for PBS and and PBSA,n respectively. However, the dispersity increased from 1.65–1.69 to 2.8–3.0. The obtained PBSA, respectively. However, the dispersity increased from 1.65–1.69 to 2.8–3.0. The obtained results results suggest the strong influence of artificial weathering on the molecular parameters of both suggest the strong influence of artificial weathering on the molecular parameters of both polymers. polymers. The number—average molar mass dependence on the degradation time during artificial The number—average molar mass dependence on the degradation time during artificial weathering was almost linear. A similar linear tendency was also observed for the dispersity, but this weathering was almost linear. A similar linear tendency was also observed for the dispersity, but molecular parameter increased with degradation time. Artificial weathering resulted in a random this molecular parameter increased with degradation time. Artificial weathering resulted in a process for the decomposition of PBS and PBSA macromolecules. Furthermore, the obtained results random process for the decomposition of PBS and PBSA macromolecules. Furthermore, the obtained corresponded to the estimated macroscopic factors of degradation, and suggested a lack of significant results corresponded to the estimated macroscopic factors of degradation, and suggested a lack of influence of addition of the adipate functional group to PBS on the degradation of PBS during significant influence of addition of the adipate functional group to PBS on the degradation of PBS artificial weathering. during artificial weathering. Obviously, different results were obtained for the biodegradation of both polymers in compost: Obviously, different results were obtained for the biodegradation of both polymers in compost: the polymer molecular weight decreased, accompanied by a reduction in the dispersity. The loss of Mn the polymer molecular weight decreased, accompanied by a reduction in the dispersity. The loss of as a result of degradation is a typical phenomenon in this process, but the decreased dispersity suggests Mn as a result of degradation is a typical phenomenon in this process, but the decreased dispersity another degradation mechanism than that observed for artificial weathering. For up to 12 weeks, suggests another degradation mechanism than that observed for artificial weathering. For up to 12 the dispersity was approximately 1.60 for both studied polymers; however, during the next interval of weeks, the dispersity was approximately 1.60 for both studied polymers; however, during the next biodegradation, an insignificant decrease was observed, and finally, after 24 weeks, the dispersities of interval of biodegradation, an insignificant decrease was observed, and finally, after 24 weeks, the dispersities of PBS and PBSA were 1.43 and 1.20, respectively. This confirms at least two mechanisms to be operating during the biodegradation in compost: the relatively slow hydrolysis of higher molar mass polymers and the faster enzymatic etching of oligomers. It is important to emphasize that enzymes are capable of etching only polymers with a sufficiently low molar mass. Therefore, this process is not random, as is the degradation during artificial weathering. Hydrolysis leads to a decrease in the overall molar mass, but enzymatic etching eliminates the oligomeric

Polymers 2018, 10, x FOR PEER REVIEW 7 of 12

fraction from the samples relatively quickly. As a consequence, both the molar mass and dispersity are reduced. Additionally, the decrease in Mn as a function of degradation time was non-linear and showed an almost exponential decrease. Moreover, the obtained results showed that the PBSA Polymers 2018, 10, 251 7 of 12 copolymer was more resistant to degradation in compost than the PBS polymer. The degradation process was also analyzed in soil (Figure 4b). Biodegradation of the studied PBSpolymers and PBSA in soil were gave 1.43 similar and 1.20, results respectively. to the Thisbiodegradation confirms at leastin compost, two mechanisms but the tochanges be operating in the duringmolecular the biodegradationparameters were in less compost: intense, the which relatively is an slow effect hydrolysis of the lower of higher content molar of microorganisms mass polymers andand thelower faster temperature enzymatic etchingof the ofprocess. oligomers. The ItM isn importantdecreased, to but emphasize without that significant enzymes arechanges capable in ofdispersity, etching only which polymers indicates with the a sufficientlyrandom degradation low molar of mass. polymer Therefore, chains, this with process uniform is not molecular random, asweight is the losses degradation in all polymer during artificialfractions. weathering. At the end Hydrolysisof the experiment, leads to the a decrease Mn values in theof PBS overall and molarPBSA mass,were 31 but and enzymatic 23 kg/mol, etching respectively. eliminates As theis clearly oligomeric shown, fraction PBSA was from more the samples resistant relatively to degradation quickly. in Assoil, a consequence,and a visible bothdecrease the molar in the mass molar and mass dispersity was observed are reduced. only Additionally, after 16 weeks. the The decrease profile in Mofn theas amolar function mass of loss degradation of PBS as time a function was non-linear of degradation and showed time anwas almost similar exponential to that for decrease. biodegradation Moreover, in thecompost. obtained The results main showed visible that change the PBSA in copolymerMn was observed was more only resistant during to degradation the initial inperiod compost of thanbiodegradation, the PBS polymer. up to 4 weeks.

Figure 4. Changes in molar mass (Mn) (black symbols) and dispersity (gray symbols) of the studied Figure 4. Changes in molar mass (Mn) (black symbols) and dispersity (gray symbols) of the samples during degradation in selected environments: (a) biodegradation in compost, (b) studied samples during degradation in selected environments: (a) biodegradation in compost, biodegradation in soil and (c) artificial weathering. (b) biodegradation in soil and (c) artiﬁcial weathering.

3.3. Analysis of the Supramolecular Structure of the Studied Materials after Degradation The degradation process was also analyzed in soil (Figure4b). Biodegradation of the studied polymersThe inpresented soil gave similarSEC-MALLS results toresults the biodegradation showed the ininfluence compost, butof various the changes conditions in the molecular on the parametersmechanism wereof polymer less intense, degradation which isand an on effect the ofchanges the lower in the content molecular of microorganisms structures of the and studied lower temperaturepolymers. To of theestimate process. the The changesMn decreased, in the supram but withoutolecular significant structures changes of the in dispersity,materials during which indicatesdegradation, the randomwide-angle degradation X-ray diffraction of polymer was chains, employed. with uniform In Figure molecular 5, examples weight of losses the inX-ray all polymerdiffraction fractions. profiles At of the both end studied of the experiment, polymers are the Mshown.n values The of PBSX-ray and diffraction PBSA were peaks 31 and at 232θ kg/mol, = 19.7°, respectively.22.1°, 22.8°, 26.2° As is and clearly 29.1°, shown, corresponding PBSA was moreto the resistant (111)/(002),̅ to degradation (012), (110), in (121 soil,)̅ andand (111) a visible planesdecrease of the inpoly(butylene) the molar mass succinate was observed monoclinic only crystal after 16lattice, weeks. were The clearly profile visible. of the The molar diffraction mass loss patterns of PBS do as anot function give much of degradation information time about was similarthe poly(butylen to that fore) biodegradation adipate crystalline in compost. structure. The The main strongest visible changediffraction in M peaksn was at observed 2θ = 17.6° only and during 21.7°, thecorresponding initial period to of the biodegradation, (002) and (110) up planes to 4 weeks. of PBA, are not detectable. This result indicates that a low content (<25% wt) of poly(butylene) adipate is present in 3.3.the Analysisstudied copolymer of the Supramolecular [22]. Structure of the Studied Materials after Degradation The presented SEC-MALLS results showed the influence of various conditions on the mechanism of polymer degradation and on the changes in the molecular structures of the studied polymers. To estimate the changes in the supramolecular structures of the materials during degradation, wide-angle X-ray diffraction was employed. In Figure5, examples of the X-ray diffraction profiles of both studied polymers are shown. The X-ray diffraction peaks at 2θ = 19.7◦, 22.1◦, 22.8◦, 26.2◦ and 29.1◦, corresponding to the (111)/(002), (012), (110), (121) and (111) planes of the poly(butylene) succinate monoclinic crystal lattice, were clearly visible. The diffraction patterns do not give much information about the poly(butylene) adipate crystalline structure. The strongest diffraction peaks at 2θ = 17.6◦ and 21.7◦, corresponding to the (002) and (110) planes of PBA, are not detectable. This Polymers 2018, 10, 251 8 of 12 result indicates that a low content (<25 wt %) of poly(butylene) adipate is present in the studied

Polymerscopolymer 2018, [1022, x]. FOR PEER REVIEW 8 of 12

Figure 5. X-ray diffraction profiles of PBS and PBSA samples with deconvolution to the amorphous Figure 5. X-ray diffraction profiles of PBS and PBSA samples with deconvolution to the amorphous and crystalline compounds. and crystalline compounds. Due to slight changes in the X-ray profiles of the polymers as a result of the degradation process,Due numerical to slight changes analysis in of the th X-raye profiles profiles was ofperforme the polymersd to estimate as a result the of quantitative the degradation influence process, of degradationnumerical analysis on the of thesupramolecular profiles was performed structures to estimateof the thestudied quantitative materials. influence For ofthis degradation analysis, deconvolutionon the supramolecular of the experimental structures ofdata the to studied the amorphous materials. halo For and this crystalline analysis, peaks deconvolution was performed of the inexperimental accordance datawith toHindeleh the amorphous and Johnson’s halo and method crystalline (Figure peaks 5). The was X-ray performed diffraction in accordance patterns were with directlyHindeleh analyzed and Johnson’s to obtain method the crystalline (Figure5 ).phase The X-raycontent, diffraction using Equation patterns (1): were directly analyzed to obtain the crystalline phase content, using Equation (1): A χ = C C + (1) ACAACA χC = (1) AC + AA where AA and AC are the integrated areas calculated under the amorphous and crystalline curves, respectively.where AA and AC are the integrated areas calculated under the amorphous and crystalline curves, respectively.In Figure 6, the degree of crystallinity of the studied polymers as a function of degradation time underIn various Figure 6conditions, the degree is ofpresented. crystallinity As is of clearl the studiedy shown, polymers the largest as achange function in the of degradation crystallinity time was observedunder various after conditionsbiodegradation is presented. in compost. As isThe clearly degree shown, of crystallinity the largest (35% change for inPBS the and crystallinity 27% for PBSA)was observed increased after to 52% biodegradation for both materials. in compost. Notably, The during degree ofbiodegradation crystallinity (35% in compost, for PBS the and polymer 27% for crystallizedPBSA) increased almost to 52%linearly. for both This materials. phenomenon Notably, was duringnot observed biodegradation in the other in compost, tested degradation the polymer conditions,crystallized where almost the linearly. degree of This crystallinity phenomenon increased was notsignificantly observed in in the the initial other period tested and degradation then did notconditions, change wheresignificantly. the degree The ofmaximum crystallinity degree increased of crystallinity significantly for inPBS the was initial approximately period and then 40% did in bothnot changeconditions, significantly. while for ThePBSA, maximum this value degree was approximately of crystallinity 37% for PBS for wasbiodegradation approximately in soil 40% and in approximatelyboth conditions, 40% while for forartificial PBSA, weathering. this value was The approximately presented results 37% forof the biodegradation change in crystallinity in soil and correlateapproximately with the 40% results for artificial presented weathering. above, in The regard presented to which results compost of the is change the most in crystallinity degrading environmentcorrelate with for the the results studied presented polymers. above, The increase in regard of crystallinity to which compost is a result is of the the most hydrolysis degrading and enzymaticenvironment degradation for the studied where polymers. amorphous The increaseparts of of polymers crystallinity are is degraded a result of first, the hydrolysis followed andby crystalline.enzymatic degradationFurthermore, where the amorphousdifferences partsbetween of polymers the studied are degraded polymers first, can followed be attributed by crystalline. to the differentFurthermore, degree the of differences crystallinity between of the thestudied studied mate polymersrials as canwell be as attributed due to the to different the different crystalline degree propertiesof crystallinity [40]. ofThe the increase studied in materials the degree as well of crystallinity as due to the during different degradation crystalline also properties affords [40an]. understandingThe increase in of the the degree observed of crystallinity macroscopic during changes degradation in the studied also samples. affords anThe understanding high crystallinity of the of theobserved samples macroscopic after biodegradation changes in in the compost studied explai samples.ns their The highability crystallinity to fragment. of theMechanical samples tests after couldbiodegradation be carried in out compost for materials explains theirwith abilitylower tocrystallinity, fragment. Mechanicali.e., biodegraded tests could in soil be carriedand aged out by for artificialmaterials weathering. with lower crystallinity, i.e., biodegraded in soil and aged by artificial weathering.

Polymers 2018, 10, 251 9 of 12 Polymers 2018, 10, x FOR PEER REVIEW 9 of 12

Polymers 2018, 10, x FOR PEER REVIEW 9 of 12

FigureFigure 6. Changes 6. Changes in the in crystallinity the crystallinity of the studiedof the studied samples samples during degradation during degradation in selected in environments: selected (a) biodegradationenvironments: (a in) biodegradation compost, (b) biodegradationin compost, (b) biodegradation in soil and (c) in artificial soil and weathering. (c) artificial weathering. Figure 6. Changes in the crystallinity of the studied samples during degradation in selected environments: (a) biodegradation in compost, (b) biodegradation in soil and (c) artificial weathering. Another characteristic feature of the structure of the studied polymers degraded under Another characteristic feature of the structure of the studied polymers degraded under different different conditions is the lattice length (d-spacing), which can be calculated using Braggs Equation conditionsAnother is the latticecharacteristic length (d-spacing),feature of the which structure can be of calculated the studied using polymers Braggs degraded Equation (2):under (2):different conditions is the lattice length (d-spacing), which can be calculated using Braggs Equation (2): λ d= (2) (2) 22sin sin θθ (2) where λ is the wavelength of the X-ray source (0.15418 nm) and θ is the angle of reflection (half of 2θ where λ is the wavelength of the X-ray source (0.154182sin θ nm) and θ is the angle of reflection (half of of the peak position). The d-spacing was calculated for the three most intense diffraction peaks, 2θ ofwhere the peak λ is the position). wavelength The of d-spacing the X-ray source was calculated (0.15418 nm) for and the θ three is themost angle intenseof reflection diffraction (half of 2 peaks,θ corresponding to the (111)/(002),̅ (012) and (110) planes. In Figure 7, the change in the d-spacing of correspondingof the peak toposition). the (11 1The)/(002), d-spacing (012) was and calculat (110) planes.ed for Inthe Figure three 7most, the intense change diffraction in the d-spacing peaks, of both studied polymers as a function of degradation time under various conditions is presented. As is bothcorresponding studied polymers to the as (111 a function)/(002),̅ (012) of degradation and (110) planes. time underIn Figure various 7, the conditionschange in the is presented.d-spacing of As is presented,both studied the polymers only visible as a changefunction in of this degradation structural time parameter under variouswas observed conditions for samples is presented. degraded As is presented, the only visible change in this structural parameter was observed for samples degraded bypresented, artificial the weathering. only visible The change presented in this results structural suggest parameter the insignificant was observed ordering for samples of the crystalline degraded by artificial weathering. The presented results suggest the insignificant ordering of the crystalline structuresby artificial during weathering. degradation, The presented which resultscorresponds suggest to the the insignificant results of orderingthe macroscopic of the crystalline changes structures during degradation, which corresponds to the results of the macroscopic changes observed observedstructures in during the materials, degradation, e.g., stress which and corresponds elongation ofto thethe sample. results Theof thestudied macroscopic materials changesbecame in thefragileobserved materials, after in artificialthe e.g., materials, stress weathering and e.g., elongation stress without and significant ofelonga the sample.tion increases of the The sample. studiedin mass The loss. materials studied Probably, materials became the effect fragilebecame of after artificialartificialfragile weathering after rainfall artificial causes without weathering hydrolytic significant without degradation, increases significant but in the mass increases more loss. important Probably,in mass factorsloss. the Probably, effect involved of artificialthe in polymereffect rainfall of causesdegradationartificial hydrolytic rainfall are degradation, causes the temperature hydrolytic but thedegradation, and more exposure important but theto more factorsUV importantradiation, involved factorswhich in polymer involvedmay degradationsupport in polymer the are the temperaturecrystallizationdegradation andare and exposure theordering temperature of to polymer UV radiation, and chains exposure which[41]. mayto UV support radiation, the crystallizationwhich may support and ordering the of polymercrystallization chains [41 and]. ordering of polymer chains [41].

Figure 7. Changes in the d-spacing of the studied samples during degradation in selected environments: (a) biodegradation in compost, (b) biodegradation in soil and (c) artificial weathering. FigureFigure 7. Changes 7. Changes in the in d-spacing the d-spacing of the studiedof the samplesstudied duringsamples degradation during degradation in selected in environments: selected (a) biodegradationenvironments: (a in) biodegradation compost, (b) biodegradation in compost, (b) biodegradation in soil and (c) in artiﬁcial soil and weathering. (c) artificial weathering.

Polymers 2018, 10, 251 10 of 12

4. Conclusions The performed investigations on the degradation of a commercially available PBS and PBSA copolymer in various environments affords a better understanding of the changes in the polymers, not only on a macroscale, but also on molecular and supramolecular scales. The results of the macroscopic measurements confirmed the better degradability of the PBSA copolymer compared with the PBS homopolymer. The most favorable degradation ENVIRONMENT was compost, which contains microorganisms and natural enzymes that support degradation. The analysis of the molecular parameters and supramolecular structures of both polymers during degradation showed that more significant changes occurred for PBSA. All of the applied environments resulted in a decrease in the molar mass and an increase in the crystallinity of both polymers, but larger changes were recorded for PBSA. The analysis of the dispersity showed that each of the selected environments involved a different mechanism for the molecular changes and degradation. In the case of artificial weathering, the degradation of the polymer chains is random, and the dispersity increases with decreasing molar mass. The obviously different mechanism of chain decomposition is observed during biodegradation in compost, where the relatively slow hydrolysis of higher molar mass polymers and faster enzymatic etching of oligomers was observed by the decrease in molar mass with decreasing dispersity. Different results were recorded during degradation in soil, where only the molar mass was changed without changes in the dispersity. The investigation of the supramolecular changes clearly showed the ordering of polymeric chains during degradation, as measured by the d-spacing. The most visible ordering was observed for the samples degraded by artificial weathering. In this case, the changes were more visible in PBS than in the PBSA copolymer.

Acknowledgments: This work was partially financed by European Regional Development Fund in the frame of “Biodegradable fibrous products” project (acronym BIOGRATEX) No. POIG 01.03.01-00-07/08-09. Part of the work was financed from funds assigned for 14-148-1-2117 statuary activity by Lodz University of Technology, Department of Material and Commodity Sciences and Textile Metrology, Poland. Author Contributions: Michał Puchalski performed the WAXD measurement, analysed all experimental data and wrote the paper; Grzegorz Szparaga performed the samples and the mechanical measurments; Tadeusz Biela performed the SEC-MALLS measurement and analysis; Sławomir Sztajnowski performed the laboratory artificial weathering of samples according to the ISO standards; Agnieszka Gutowska performed the laboratory biodegradation of samples in soil and compost according to the ISO standards; Izabella Krucińskadesigned the experiment, the leader of the Biogratex Project. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

1. Tokiwa, Y.; Calabia, B.P.; Ugwu, C.U.; Aiba, S. Biodegradability of Plastics. Int. J. Mol. Sci. 2009, 10, 3722–3742. [CrossRef][PubMed] 2. Mochizuki, M. Properties and Application of Aliphatic Polyester Products. Biopolym. Online 2005, 4.[CrossRef] 3. Xu, J.; Guo, B.H. Microbial Succinic Acid, Its Polymer Poly(butylene succinate), and Applications. In Plastics from Bacteria. Microbiology Monographs; Chen, G.Q., Ed.; Springer: Berlin, Germany, 2010; Volume 14, pp. 347–388, ISBN 978-3-642-03286-8. 4. Jacquel, N.; Freyermouth, F.; Fenouillot, F.; Rousseau, A.; Pascault, J.P.; Fuertes, P.; Saint-Loup, R. Synthesis and properties of poly(butylene succinate): Efﬁciency of different transesteriﬁcation catalysts. J. Polym. Sci. A Polym. Chem. 2011, 49, 5301–5312. [CrossRef] 5. Bikiaris, D.N.; Achilias, D.S. Synthesis of poly(alkylene succinate) biodegradable polyesters, Part II: Mathematical modelling of the polycondensation reaction. Polymer 2008, 49, 3677–3685. [CrossRef] Polymers 2018, 10, 251 11 of 12

6. Zorba, T.; Chrissafis, K.; Paraskevopoulos, K.M.; Bikiaris, D.N. Synthesis, characterization and thermal degradation mechanism of three poly(alkylene adipate)s: Comparative study. Polym. Degrad. Stab. 2007, 92, 222–230. [CrossRef] 7. Tserki, V.; Matzinos, P.; Pavlidou, E.; Vachliotis, D.; Panayiotou, C. Biodegradable aliphatic polyesters. Part I. Properties and biodegradation of poly(butylene succinate-co-butylene adipate). Polym. Degrad. Stab. 2006, 91, 367–376. [CrossRef] 8. Nikolic, M.S.; Djonlagic, S. Synthesis and characterization of biodegradable poly(butylene succinate-co- butylene adipate)s. Polym. Degrad. Stab. 2001, 74, 263–270. [CrossRef] 9. Fujimaki, T. Processability and properties of aliphatic polyesters, ‘BIONOLLE’, synthesized by polycondensation reaction. Polym. Degrad. Stab. 1998, 59, 209–214. [CrossRef] 10. Ichikawa, Y.; Mizukoshi, T. Bionolle (Polybutylenesuccinate). In Synthetic Biodegradable Polymers; Advances in Polymer Science; Rieger, B., Künkel, A., Coates, G., Reichardt, R., Dinjus, E., Zevaco, T., Eds.; Springer: Berlin, Germany, 2011; Volume 245, pp. 285–313, ISBN 978-3-642-27153-3. 11. Showa Denko Europe. Available online: http://www.showa-denko.com/news/bionolle-the-pioneer-in- biodegradable (accessed on 20 January 2018). 12. McKinlay, J.B.; Vieille, C.; Zeikus, J.G. Prospects for a bio-based succinate industry. Appl. Microbiol. Biotechnol. 2007, 76, 727–740. [CrossRef][PubMed] 13. Bechthold, I.; Bretz, K.; Kabasci, S.; Kopitzky, R.; Springer, A. Succinic Acid: A New Platform Chemical for Biobased Polymers from Renewable Resources. Chem. Eng. Technol. 2008, 31, 647–654. [CrossRef] 14. Debuissy, T.; Pollet, E.; Avérous, L. Synthesis of potentially biobased copolyesters based on adipic acid and butanediols: Kinetic study between 1,4- and 2,3-butanediol and their influence on crystallization and thermal properties. Polymer 2016, 99, 204–213. [CrossRef] 15. Lichocik, M.; Owczarek, M.; Miros, P.; Guzińska, K.; Gutowska, A.; Ciechańska, D.; Krucińska, I.; Siwek, P. Impact of PBSA (Bionolle) Biodegradation Products on the Soil Microbiological Structure. Fibres Text. East. Eur. 2012, 20, 179–185. 16. Siwek, P.; Libik, A.; Kalisz, A.; Zawiska, I. The effect of biodegradable nonwoven direct covers on yield and quality of winter leek. Folia Hortic. 2013, 25, 61–65. [CrossRef] 17. Bhatia, A.; Gupta, R.; Bhattacharya, S.; Choi, H. Compatibility of biodegradable poly (lactic acid) (PLA) and poly (butylene succinate) (PBS) blends for packaging application. Korea Aust. Rheol. J. 2013, 19, 125–131. 18. Jompang, L.; Thumsorn, S.; On, J.W.; Surin, P.; Apawet, C.; Chaichalermwong, T.; Kaabbuathong, N.; O-Charoen, N.; Srisawat, N. Poly(lactic acid) and Poly(butylene succinate) Blend Fibers Prepared by Melt Spinning Technique. Energy Procedia 2013, 34, 493–499. [CrossRef] 19. Twarowska-Schmidt, K.; Tomaszewski, W. Evaluation of the Suitability of Selected Aliphatic Polyester Blends for Biodegradable Fibrous Materials with Improved Elasticity. Fibres Text. East. Eur. 2008, 16, 9–13. 20. Xu, J.; Guo, B.-H. Poly(butylene succinate) and its copolymers: Research, development and industrialization. Biotechnol. J. 2010, 5, 1149–1163. [CrossRef][PubMed] 21. Lai, S.-M.; Huang, C.-K.; Shen, H.-F. Preparation and properties of biodegradable poly(butylene succinate)/ starch blends. J. Appl. Polym. Sci. 2005, 97, 257–264. [CrossRef] 22. Salehiyan, R.; Ray, S.S.; Bandyopadhyay, J.; Ojijo, V. The Distribution of Nanoclay Particles at the Interface and Their Influence on the Microstructure Development and Rheological Properties of Reactively Processed Biodegradable Polylactide/Poly(butylene succinate) Blend Nanocomposites. Polymers 2017, 9, 350. [CrossRef] 23. Gigli, M.; Negroni, A.; Zanaroli, G.; Lotti, N.; Fava, F.; Munari, A. Environmentally friendly PBS-based copolyesters containing PEG-like subunit: Effect of block length on solid-state properties and enzymatic degradation. React. Funct. Polym. 2013, 73, 764–771. [CrossRef] 24. Tokiwa, Y.; Calabia, B.P. Biodegradability and Biodegradation of Polyesters. J. Polym. Environ. 2007, 15, 259–267. [CrossRef] 25. Bautista, M.; de Ilarduya, A.M.; Alla, A.; Muñoz-Guerra, S. Poly(butylene succinate) Ionomers with Enhanced Hydrodegradability. Polymers 2015, 7, 1232–1247. [CrossRef] 26. Ding, M.; Zhang, M.; Yang, J.; Qiu, J.-H. Study on the Enzymatic Degradation of Aliphatic Polyester–PBS and Its Copolymers. J. Appl. Polym. Sci. 2012, 124, 2902–2907. [CrossRef] 27. Kimble, L.D.; Bhattacharyya, D. In Vitro Degradation Effects on Strength, Stiffness, and Creep of PLLA/PBS: A Potential Stent Material. Int. J. Polym. Mater. Polym. Biomater. 2015, 64, 299–310. [CrossRef] Polymers 2018, 10, 251 12 of 12

28. Shih, Y.-F. Thermal Degradation and Kinetic Analysis of Biodegradable PBS/Multiwalled Carbon Nanotube Nanocomposites. J. Polym. Sci. B 2009, 47, 1231–1239. [CrossRef] 29. Cho, K.; Lee, J.; Kwon, K. Hydrolytic Degradation Behavior of Poly(butylene succinate)s with Different Crystalline Morphologies. J. Appl. Polym. Sci. 2001, 79, 1025–1033. [CrossRef] 30. Puchalski, M.; Siwek, P.; Biela, T.; Sztajnowski, S.; Chrzanowski, M.; Kowalska, S.; Krucińska,I. Influence of the homehold composting conditions on the structural changes of polylactide spun-bonded nonwovens during degradation. Text. Res. J. 2017, 87, 2541–2549. [CrossRef] 31. Lindström, A.; Albertsson, C.-A.; Hakkarainen, M. Quantitative determination of degradation products an effective means to study early stages of degradation in linear and branched poly(butylene adipate) and poly(butylene succinate). Polym. Degrad. Stab. 2004, 83, 487–493. [CrossRef] 32. Bikiaris, D.N.; Papageorgiou, G.Z.; Giliopoulos, D.J.; Stergiou, C.A. Correlation between Chemical and Solid-State Structures and Enzymatic Hydrolysis in Novel Biodegradable Polyesters. The Case of Poly(propylene alkanedicarboxylate)s. Macromol. Biosci. 2008, 8, 728–740. [CrossRef][PubMed] 33. Bikiaris, D.N.; Papageorgiou, G.Z.; Achilias, D.S. Synthesis and comparative biodegradability studies of three poly(alkylene succinate)s. Polym. Degrad. Stab. 2006, 91, 31–43. [CrossRef] 34. Papageorgiou, G.Z.; Nanaki, S.G.; Bikiaris, D.N. Synthesis and characterization of novel poly(propylene terephthalate-co-adipate) biodegradable random copolyesters. Polym. Degrad. Stab. 2010, 95, 627–637. [CrossRef] 35. Gutowska, A.; Jó´zwicka, J.; Sobczak, S.; Tomaszewski, W.; Sulak, K.; Miros, P.; Owczarek, M.; Szalczyńska, M.; Ciechańska, D.; Krucińska, I. In-Compost Biodegradation of PLA Nonwovens. Fibres Text. East. Eur. 2014, 22, 99–106. 36. Rabiej, M. Application of the genetic algorithms and multi-objective optimisation to the resolution of X-ray diffraction curves of semicrystalline polymers. Fibres Text. East. Eur. 2003, 11, 83–87. 37. Papadimitriou, S.A.; Papageorgiou, G.Z.; Bikiaris, D.N. Crystallization and enzymatic degradation of novel poly(e-caprolactone-co-propylene succinate) copolymers. Eur. Polym. J. 2008, 44, 2356–2366. [CrossRef] 38. Chandra, R.; Rustgi, R. Biodegradable Polymers. Prog. Polym. Sci. 1998, 23, 1273–1335. [CrossRef] 39. Stepto, R.F.T.; Gilbert, R.G.; Hess, M.; Jenkins, A.D.; Jones, R.G.; Kratochvíl, P. Dispersity in Polymer Science. Pure Appl. Chem. 2009, 81, 351–353. [CrossRef] 40. Papageorgiou, G.Z.; Bikiaris, D.N. Crystallization and melting behavior of three biodegradable poly(alkylene succinates). A comparative study. Polymer 2005, 46, 12081–12092. [CrossRef] 41. Zhang, Y.; Xu, J.; Guo, B. Photodegradation behavior of poly(butylene succinate-co-butyleneadipate)/ZnO nanocomposites. Colloids Surf. A 2016, 489, 173–181. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).