polymers Article The Thermal and Mechanical Properties of Poly(ethylene-co-vinyl acetate) Random Copolymers (PEVA) and its Covalently Crosslinked Analogues (cPEVA) Ke Wang 1 and Qibo Deng 2,3,* 1 School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China; [email protected] 2 Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China 3 Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China * Correspondence: [email protected]; Tel.: +86-22-6021-5388 Received: 21 May 2019; Accepted: 12 June 2019; Published: 17 June 2019 Abstract: The thermal and mechanical properties of poly(ethylene-co-vinyl acetate) random copolymers (PEVA) and its covalently crosslinked analogues (cPEVA) were controlled by the overall crystallinity of the polymer networks. The cPEVAs with different VA-content were synthesized by thermally-induced crosslinking of linear PEVA with dicumyl peroxide (DCP). This work was mainly concerned with the effect of vinyl acetate (VA) content on the crosslinking density, thermal and mechanical properties of PEVAs and cPEVAs, respectively. The chemical composition was analyzed by thermogravimetric analysis and 1H-NMR. The thermal and mechanical properties of PEVAs and cPEVAs have been studied through a series of conventional analytical methods, including gel content determination, different scanning calorimetry, thermogravimetric analysis, dynamic mechanical thermal analysis and traditional mechanical measurements. The experimental results show that the thermal and mechanical properties of PEVAs and cPEVAs increase with decreasing the VA-content. A broad melting transition with a DTm in the range from 78 ◦C to 95 ◦C was observed for all polymer networks. Keywords: random copolymers; mechanical property; thermal property; covalent crosslinking 1. Introduction Shape memory polymers (SMP) are materials which can be deformed and fixed in a temporary shape, from which they recover their original, permanent shape when being exposed to a certain external stimulus, such as heat, light, electricity or magnetic field [1–4]. The emerging field of SMP has attracted tremendous interest due to its various potential applications covering smart packaging [5], heat-shrink tubing, deployable structures and microdevices [6], intelligent drug releasing systems and medical devices for minimally invasive surgery [7–13], etc. In the last decade, shape-memory semi-crystalline polymers with covalent crosslinking, e.g., degradable SMPs containing poly("-caprolactone) (PCL) switching segments [12,14,15] or covalent crosslinked poly(ethylene-co-vinyl acetate) (cPEVA) using polyethylene (PE) switching segments [16], have been widely studied and become popular in biomedical application. Poly(ethylene-co-vinyl acetate) [17] is a random copolymer consisting of semi-crystalline polyethylene (PE) segments and amorphous poly(vinyl acetate) (PVA) segments. Figure1 shows the chemical structure of monomers and PEVA copolymer. PEVA is a thermoplastic polymer extensively Polymers 2019, 11, 1055; doi:10.3390/polym11061055 www.mdpi.com/journal/polymers Polymers 2019, 11, x FOR PEER REVIEW 2 of 18 Poly(ethylene‐co‐vinyl acetate) [17] is a random copolymer consisting of semi‐crystalline Polymers 2019, 11, 1055 2 of 18 polyethylene (PE) segments and amorphous poly(vinyl acetate) (PVA) segments. Figure 1 shows the chemical structure of monomers and PEVA copolymer. PEVA is a thermoplastic polymer extensively used in didifferentfferent fields,fields, such as flexibleflexible packaging, footwear, hot met adhesives and cable sheathing. PEVA isis also also considered considered as aas good a good candidate candidate for biomedical for biomedical application application because because of its ease of ofits handling ease of andhandling processing, and processing, biocompatibility biocompatibility or drug delivery or drug capability delivery capability [18,19]. [18,19]. Figure 1.1. ChemicalChemical structure structure of of monomers monomers and and poly(ethylene- poly(ethyleneco-vinyl‐co‐vinyl acetate) acetate) random random copolymers copolymers (PEVA). (PEVA). The physical and mechanical properties of PEVA are influenced by the crystallinity of copolymerThe physical [20–23 and], which mechanical can be properties adjusted of by PEVA the variation are influenced of VA-content by the crystallinity [17]. The of copolymer formation of[20–23], the crystalline which can structure be adjusted is due by the to the variation organized of VA arrangements‐content [17]. of The linear formation polyethylene of the crystalline chains in PEVA.structure As is the due increase to the organized of VA-content, arrangements the stereoregularity of linear polyethylene of polymer chains chains in PEVA. reduces, As resultingthe increase in theof VA decrease‐content, of thethe crystallinitystereoregularity of PE of segments. polymer Therefore,chains reduces, both theresulting melting in temperaturethe decrease and of thethe storagecrystallinity modules of PE of segments. PEVAs can Therefore, be reduced both [17 the]. Brogly melting et al.temperature [20] and Arsac and the et al. storage [21] reported modules that of thePEVAs melting can temperaturebe reduced (T[17].m) ofBrogly different et PEVAsal. [20] decreasesand Arsac with et theal. reduction[21] reported of crystallinity, that the melting which resultstemperature either (T fromm) of imperfection different PEVAs or variation decreases of with crystals. the reduction Sung et al.of crystallinity, [17] also suggested which results that storage either modulusfrom imperfection of PEVAs declinesor variation with of augment crystals. of Sung VA-content et al. at[17] temperatures also suggested above thatTg storagedue to the modulus reduction of ofPEVAs crystallinity, declines which with couldaugment reinforce of VA the‐content mechanical at temperatures property of PEVAs.above Tg due to the reduction of crystallinity,On the other which hand, could crosslinking reinforce the density mechanical also plays property an important of PEVAs. role in adjusting the material properties.On the In other generally, hand, thecrosslinking increase of density crosslinking also plays density an important can result inrole the in increase adjusting of the material propertiesproperties. (modulus,In generally, hardness, the increase resilience, of crosslinking and abrasion density resistance) can result in whereas the increase the decrease of the material of the elongationproperties at(modulus, break, heat hardness, build-up, resilience, and stress relaxation.and abrasion Recent resistance) reports havewhereas shown the that decrease PEVAs canof the be crosslinked,elongation at either break, by heat the exposure build‐up, of and the stress polyethylene relaxation. homopolymers Recent reports to high-energy have shown ray that (e.g., PEVAs electron can beambe crosslinked, or γ-ray), oreither by chemical by the exposure crosslinking of the (e.g., polyethylene peroxide or silane homopolymers crosslinking) to [high17,24‐energy,25]. Li etray al. (e.g., [16] andelectron Sung beam et al. or [17 γ‐] haveray), provedor by chemical the influence crosslinking of crosslinking (e.g., peroxide degree of or cPEVA silane oncrosslinking) its dynamic [17,24,25]. modulus aroundLi et al. the[16] melting and Sung temperature. et al. [17] have Yao et proved al. [18] the have influence also suggested of crosslinking that the degree thermal of stability cPEVA can on beits enhanceddynamic modulus considerable around by the the crosslinking melting temperature. of PEVA. Crosslinked Yao et al. PEVA[18] have materials also suggested have been usedthat the for variousthermal fields stability such can as photovoltaic be enhanced modules, considerable insulation by the materials, crosslinking cables, of damping PEVA. mattressCrosslinked for railroad PEVA crossties,materials and have shoe been soles used [26 –for31]. various To improve fields its such mechanical as photovoltaic strength and modules, thermal insulation resistance properties,materials, PEVAcables, is damping generally mattress cured by for peroxide railroad [32 crossties,–35], such and as dicumylshoe soles peroxide [26–31]. (DCP). To improve The mechanism its mechanical of the crosslinkingstrength and is thermal due to the resistance formation properties, of the radiacal PEVA of is the generally DCP [36 cured,37]. The by DCPperoxide crosslinking [32–35], reactionsuch as wasdicumyl carried peroxide out preferably (DCP). The on bothmechanism VA-segments of the andcrosslinking PE-segments. is due How to the the formation –CH2 and of –CH the radiacal reacted andof the how DCP much [36,37]. of them The reactedDCP crosslinking can be characterized reaction was by Ramancarried spectroscopy.out preferably on both VA‐segments and ThisPE‐segments. study focuses How to the examine –CH2 theand effect –CH of reacted VA content and onhow the much properties of them of PEVA reacted and cPEVA,can be whichcharacterized include by the Raman crosslink spectroscopy. density, thermal and mechanical characteristics. These were examined throughThis a study series focuses of conventional to examine analytical the effect methods, of VA content including on the gel properties content determination, of PEVA and di cPEVA,fferent scanningwhich include calorimetry, the crosslink thermogravimetric