Elsevier

Polymer Testing

Volume 90, October 2020, 106740
Polymer Testing

Material Properties
Multiscale structural characterization of biocompatible poly(trimethylene carbonate) networks photo-cross-linked in a solvent

https://doi.org/10.1016/j.polymertesting.2020.106740Get rights and content

Highlights

  • Biocompatible PTMC have been synthesized in solvent with various PTMC concentrations.

  • DMA and Time Domain 1H DQ NMR relate well to network crosslink density & morphology.

  • PTMC crosslink density varies linearly with polymer content in the reactive media.

  • Physical entanglements reinforces PTMC of low polymer content and high macromer MW.

  • A Mechanical testing, DMA, and Time Domain 1H DQ NMR approach was robustly used.

Abstract

Poly(trimethylene carbonate) (PTMC) polymeric networks are biocompatible materials with potential biomedical applications. By combining Dynamic Mechanical Analysis (DMA), Solid State Nuclear Magnetic Resonance (NMR), and tensile testing, it was possible to fully characterize the inner structure and its relationship with the macroscopic properties of photo-crosslinked PTMC materials in a solvent medium. PTMC prepared from macromer with various molecular weights (3 kg/mol, 18 kg/mol, and 32 kg/mol) and with various polymer concentrations within the reactive media were analyzed, with the variation of thermomechanical properties and NMR signal decay characterized as a function of both aforementioned synthesis parameters. DMA and solid state Double Quantum (DQ) 1H NMR investigations demonstrated that the network crosslink density is directly related to the macromer molar mass and polymer concentration. More interestingly, tensile tests confirmed that mechanical behavior depended on the materials’ inner structure, notably their crosslink density. Specifically for the 18 kg/mol PTMC networks, dangling and free chains reinforced the network, exemplified by higher Young’s modulus E values. This multiscale investigation provides a promising and precise approach to tailor the macroscopic behavior of PTMC materials, by controlling their specific inner network structural morphologies during synthesis.

Introduction

Biodegradable polymer networks have great potential in biomedical applications due to their biocompatibility and biodegradability [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. More specifically, tough elastomeric polymer networks are of interest as bulk materials [12], [16], [17], [18], [19], [20], [21]. Elastomeric polymer networks can be designed to exhibit mechanical properties required for their intended application, especially soft tissues [22]. Much work has been done to improve the toughness of the elastomeric networks, but it remains a challenge to obtain elastomers that are both degradable and tough [18].

To improve the toughness of elastomeric networks, the growth of micro-cracks must be hindered [23]. This has been extensively studied for both natural and synthetic elastomers, though many of these studies have been performed with non-degradable elastomers such as polyisoprene and poly(dimethyl siloxane) (PDMS) [24], [25], [26], [27], [28], [29]. The formation of micro-cracks in these networks was slowed down by introducing crystallizable domains into the polymer network, crosslinking in solvent, and crosslinking chains with bimodal chain length distributions.

Tough, biodegradable elastomeric networks have been preparedvia the photo-crosslinking of methacrylate-functionalized PTMColigomers (PTMC macromers) and acrylated poly(e- caprolactone-co-D,L-lactide) (P(CL-co-DLLA)) oligomers [16], [18], [30]. Such networks are formed by adding a photo-initiator which forms radical species upon light irradiation [31]. These radical species can initiate polymerization of the (meth)acrylate end-groups of the macromers, creating poly(methacrylate) kinetic chains as the networks are formed. In the case of PTMC-networks, toughness increased with macromer molecular weight [16]. In addition, the preparation of bimodal PTMC networks resulted in a significant increase in toughness [19]. For both PTMC- and P(CL-co-DLLA) macromers, crosslinking in solution resulted in less rigid networks that demonstrated increasing elongation with decreasing macromer concentrations, resulting in superior elastomer toughness [18], [30].

It has been suggested that the presence of solvent during crosslinking results in disentangled chains prior to crosslinking formation, which in turn leads to simpler topologies in the networks [25]. The crosslinks of such networks are more likely to be both spatial and topological neighbors as compared to networks prepared in bulk. In addition, networks prepared in solution have fewer chain-junction and inter-chain entanglements. The polymer chains in these networks are less-firmly embedded in the network structure; subsequently these networks deform more non-affinely. In order to fully understand the influence of crosslinking in solution on the macroscopic properties of elastomeric networks, the thermomechanical behavior and structural morphology of these networks need to be fully studied.

To facilitate this, a macroscopic experimental approach combining tensile testing, DSC and DMA analyses, and Time Domain 1H Double Quantum DQ NMR was undertaken in this work. Tensile tests yielded the macroscopic mechanical resistance of the studied PTMC samples, while DSC and DMA allowed the evolution of molecular mobility to be analyzed via observation of the glass transition temperature. Moreover, DMA allowed the characterization of the mechanical crosslink density of PTMC networks with varying molecular weight and solvent concentrations [32], [33]. These studies were complemented by Time Domain 1H DQ NMR. This specific technique has been principally used to characterize the structure, morphological organization and molecular mobility of polymeric networks [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]. Double-Quantum DQ 1H sequences have been proven to effectively characterize elastomeric-like polymer networks, specifically their molecular mobility, crosslink density vC, and chain defect concentration wDEF [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], in addition to the evolution of these properties with temperature [58], [59], chemical [60], [61], [62] and physical modifications [63], and thermal aging [64], [65]. This is made possible due to the ability of this technique to discriminate dynamical and structural effects, permitting semi-local structural features of networks to be recovered from local dynamical measurements.

More extensively, Time Domain 1H DQ NMR has been efficiently used in combination with DMA analyses to study the relationship between the structure of PTMC networks of various macromer molar masses, and their macroscopic thermomechanical behavior [66]. Therefore, this work is a logical evolution of this previous study. By applying the same approach to analyze the influence of polymer concentration during PTMC network formation, we aim to deepen the understanding of the influence of intrinsic polymer structure on the macroscopic behavior of PTMC networks, which in turn will provide deeper insight regarding how to chemically tailor network structure, so as to better regulate and adjust PTMC specific macroscopic properties. PTMC networks were chosen for this study, as such networks are extensively studied for a range of biomedical applications requiring different properties, such as intervertebral disks [67], meniscus implants [17] and bone implants [20]. Its biocompatibility has been shown previously with, among others, synovium derived cells [68], mesenchymal stem cells [69], and chondrocytes [70] in vitro and in several studies in vivo [68], [71], [72].

Section snippets

Materials

Trimethylene carbonate (TMC) monomer was purchased fromHuizhou ForYou Medical Devices Co. (China). Hydroquinone,methacrylic anhydride, tin(II) 2-ethylhexanoate (Sn(Oct)2), Trimethylol propane (TMP), and triethylamine were purchased from Sigma (USA) and used as received. Dichloromethane and chloroform were obtained from Merck (Germany), and d-chloroform was purchased from VWR. Ethanol was obtained from Altia oyj (Finland). TPO-L (2,4,6-trimethylbenzoylphenyl phosphinate) was obtained from

Swelling characterization

The volume degree of swelling q was determined in triplicate at room temperature by swelling rectangular shaped specimens (5 × 5 × 1 mm3) in chloroform for 24 h, which was sufficient time to reach solvent sorption equilibrium. The q was calculated from Eq. (1) q=1+mswollenmdrymdryρpρswhere mswollen is the mass of the swollen networks, mdry the mass of the networks after drying, and ρp and ρs the densities of PTMC (=1.31 g/cm3) [16] and chloroform (=1.48 g/cm3), respectively.

DSC analysis

The thermal

Results and discussion

Three-armed PTMC macromers were synthesized via the ring-opening polymerization of TMC into PTMC oligomers followed by functionalization with methacrylic anhydride. By adjusting the monomer-to-initiator ratio, oligomers with three different molecular weights were obtained. Table 1 shows the obtained oligomer molecular weights as confirmed by 1H-NMR. The subsequent functionalization resulted in macromers with a degree of functionalization of 86%. In Figure SI.4 (Support Information), a typical

Conclusion

This investigation has highlighted the advantages of combining a multiscale experimental approach to better understand the macroscopic mechanical properties of PTMC networks from their intrinsic structural morphology. By combining tensile testing, DMA analyses and 1H DQ NMR measurements it was shown that PTMC crosslink density depended on the polymer content during the crosslinking reaction; the lower the polymer content, the lower the crosslink density value. This was systematically observed

CRediT authorship contribution statement

Bas van Bochove: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Visualization, Investigation, Supervision, Writing - review & editing. Steve Spoljaric: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Visualization, Investigation, Supervision, Writing - review & editing. Jukka Seppälä: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Visualization, Investigation, Supervision, Writing - review

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work made use of Aalto University Bioeconomy Facilities. The authors are deeply grateful towards Cédric Lorthioir for sharing the optimized DQ 1H Solid State NMR pulse sequence used in this work and towards Paul Sotta for fruitful discussions.

Funding

This project was funded by an Aalto University postdoctoral researcher project in synthesis of novel biopolymers (Finland) and by the Centre National de la Recherche Scientifique CNRS (France) . The authors declare no conflict of interests.

References (78)

  • FecheteR. et al.

    Anisotropy of collagen fiber orientation in sheep tendon by 1H double-quantum-filtered NMR signals

    J. Magn. Res.

    (2003)
  • SaalwächterK.

    Proton multiple-quantum NMR for the study of chain dynamics and structural constraints in polymeric soft materials

    Prog. Nucl. Magn. Reson. Spectrosc.

    (2007)
  • MansillaM. et al.

    Effect of entanglements in the microstructure of cured NR/SBR blends prepared by solution and mixing in a two-roll mill

    E. Polym. J.

    (2016)
  • PirvuT. et al.

    A combined biomaterial and cellular approach for annulus fibrosus rupture repair

    Biomaterials

    (2015)
  • BatE. et al.

    Ultraviolet light crosslinking of poly(trimethylene carbonate) for elastomeric tissue engineering scaffolds

    Biomaterials

    (2010)
  • GuillaumeO. et al.

    Surface-enrichment with hydroxyapatite nanoparticles in stereolithography-fabricated composite polymer scaffolds promotes bone repair

    Acta Biomater.

    (2017)
  • GuillaumeO. et al.

    Orbital floor repair using patient specific osteoinductive implant made by stereolithography

    Biomaterials

    (2020)
  • ZhuK. et al.

    Synthesis, properties, and biodegradation of poly (1,3-trimethylene carbonate)

    Macromolecules

    (1991)
  • AlbertsonA.-C. et al.

    Homopolymerization of 1,8dioxan-2-one to high molecular weight poly(trimethylene carbonate)

    J. Macromol. Sci.

    (1992)
  • AlbertsonA.-C. et al.

    Influence of molecular structure on the degradation mechanism of degradable polymers: In vitro degradation of poly(trimethylene carbonate), poly(trimethylene carbonate-co-caprolactone), and poly(adipic anhydride)

    J. Appl. Polym. Sci.

    (1995)
  • WangH. et al.

    Synthesis and characterization of ABA-type block copolymer of poly(trimethylene carbonate) with poly(ethylene glycol): Bioerodible copolymer

    J. Polym. Sci. Part A: Polym. Chem.

    (2000)
  • PêgoA. et al.

    In vivo behavior of poly(1,3-trimethylene carbonate) and copolymers of 1,3-trimethylene carbonate with D,L-lactide or caprolactone: Degradation and tissue response

    J. Biomed. Mater. Res.

    (2003)
  • ZhangY. et al.

    Synthesis and drug release behavior of poly (trimethylene carbonate)–poly (ethylene glycol)–poly (trimethylene carbonate) nanoparticles

    Biomaterials

    (2005)
  • SansonC. et al.

    Biocompatible and biodegradable poly(trimethylene carbonate)-b-poly(l-glutamic acid) polymersomes: Size control and stability

    Langmuir

    (2010)
  • SansonC. et al.

    Temperature responsive poly(trimethylene carbonate)-block-poly(l-glutamic acid) copolymer: Polymersomes fusion and fission

    Soft Matter

    (2010)
  • FukushimaK.

    Poly(trimethylene carbonate)-based polymers engineered for biodegradable functional biomaterials

    Biomater. Sci.

    (2016)
  • van BochoveB. et al.

    Preparation of designed poly(trimethylene carbonate) meniscus implants by stereolithography: Challenges in stereolithography

    Macromol. Biosience

    (2016)
  • van BochoveB. et al.

    Photo-crosslinked elastomeric bimodal poly(trimethylene carbonate) networks

    Macromol. Mater. Eng.

    (2019)
  • GevenM. et al.

    Fabrication of patient specific composite orbital floor implants by stereolithography

    Polym. Advan. Technol.

    (2015)
  • Schüller-RavooS. et al.

    Preparation of a designed poly(trimethylene carbonate) microvascular network by stereolithography

    Adv. Healthc. Mat.

    (2014)
  • AmsdenB.

    Curable, biodegradable elastomers: Emerging biomaterials for drug delivery and tissue engineering

    Soft Matter

    (2007)
  • SmithT.L.

    Strength of elastorners—A perspective

    Polym. Eng. Sci.

    (1977)
  • Hoo FattM.S. et al.

    Fracture parameters for natural rubber under dynamic loading

    Strain

    (2011)
  • PremachandraJ.K. et al.

    Effects of dilution during crosslinking on strain-induced crystallization in cis-1,4-polyisprene networks. I. Experimental results

    J. Macromol. Sci. A

    (2002)
  • JohnsonR.M. et al.

    Properties of poly(dimethylsiloxane) networks prepared in solution, and their use in evaluating the theories of rubberlike elasticity

    Macromolecules

    (1972)
  • MarkJ.E.

    Improved elastomers through control of network chain-length distributions

    Rubber Chem. Technol.

    (1999)
  • UrayamaK.

    Network topology – mechanical properties relationships of model elastomers

    Polym. J.

    (2008)
  • AmsdenB.G. et al.

    Synthesis and characterization of a photo-cross-linked biodegradable elastomer

    Biomacromolecules

    (2004)
  • MelchelsF.P.W. et al.

    Photo-cross-linked poly(dl-lactide)-based networks. structural characterization by HR-MAS NMR spectroscopy and hydrolytic degradation behavior

    Macromolecules

    (2010)
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