Glass Transition Behavior of Wet Polymers

materials

Article Glass Transition Behavior of Wet Polymers

Hai Li 1 and Rui Xiao 2,*

1 College of Mechanics and Materials, Hohai University, Nanjing 210098, China; [email protected] 2 Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, China * Correspondence: [email protected]

Abstract: We have performed a systematical investigation on the glass transition behavior of amorphous polymers with different solvent concentrations. Acrylate-based amorphous polymers are synthesized and treated by isopropyl alcohol to obtain specimens with a homogenous solvent distribution. The small strain dynamic mechanical tests are then performed to obtain the glass transition behaviors. The results show that the wet polymers even with a solvent concentration of more than 60 wt.% still exhibit a glass transition behavior, with the glass transition region shifting to lower temperatures with increasing solvent concentrations. A master curve of modulus as a function of frequency can be constructed for all the polymer–solvent systems via the time–temperature superposition principle. The relaxation time and the breadth of the relaxation spectrum are then obtained through ﬁtting the master curve using a fractional Zener model. The results indicate that the breadth of the relaxation spectrum has been greatly expanded in the presence of solvents, which has been rarely reported in the literature. Thus, this work can potentially advance the fundamental understanding of the effects of solvent on the glass transition behaviors of amorphous polymers.

Keywords: glass transition; viscoelastic; relaxation spectrum; fractional Zener model

Citation: Li, H.; Xiao, R. Glass 1. Introduction Transition Behavior of Wet Polymers. Amorphous polymers exhibit a glass transition behavior, across which the thermo- Materials 2021, 14, 730. https:// mechanical properties, such as modulus, heat capacity, coefficient of thermal expansion, doi.org/10.3390/ma14040730 etc., all exhibit a tremendous change. Thus, it is important to investigate glass transition behavior of amorphous polymer systems, which mainly depends on the chemical composi- Academic Editor: Francesca Lionetto tion of polymers. However, it can also be affected by other factors, such as aging [1] and Received: 6 January 2021 plasticization effects [2–6]. Accepted: 1 February 2021 Plasticizers are known for the ability to modify mechanical behaviors of polymers. Published: 4 February 2021 By embedding in and distributing through polymer matrix, the plasticizers can separate polymer chains and weaken the intermolecular interaction of polymers, resulting in an Publisher’s Note: MDPI stays neutral increase in mobility of polymer chains and consequently a decrease in glass transition with regard to jurisdictional claims in temperature (Tg)[7,8]. The effects of a plasticizer depend on the concentration as well as its published maps and institutional affil- compatibility with the polymer matrix [9]. For example, Da Silva et al. [10] adopted X-ray iations. diffraction, differential scanning calorimetry (DSC) and Fourier transform infrared spec- troscopy to characterize the change in mechanical properties of polyvinylchloride (PVC) reinforced with a natural polymeric plasticizer. The plasticizer has good compatibility with the PVC and can considerably modify the mechanical properties of PVC. Copyright: © 2021 by the authors. Solvent is also widely used as a plasticizer. It has been shown that that solvent can Licensee MDPI, Basel, Switzerland. reduce Tg of amorphous polymers. For example, Kawai and Hagura [11] investigated This article is an open access article the Tg of carbohydrate polymer solution systems using DSC. The result showed that the distributed under the terms and glass transition behaviors of carbohydrate polymer solution systems can be classified conditions of the Creative Commons into three regions according to the concentration of solution, and that the glass transition Attribution (CC BY) license (https:// region was much broader at an intermediate solution concentration than at high and low creativecommons.org/licenses/by/ 4.0/). concentrations. Huang et al. [2–4] found that Tg of polyurethane polymers can be reduced

Materials 2021, 14, 730. https://doi.org/10.3390/ma14040730 https://www.mdpi.com/journal/materials Materials 2021, 14, 730 2 of 13

dramatically after treatment by water, and the key role behind the phenomenon is that bound water acts as a plasticizer. In addition, Xiao et al. [5] used organic solvents to tune the glass transition region of amorphous polymers. The effect of solvent on the glass transition region has been employed to achieve solvent responsive shape-memory effects, which has received extensive investigation in recent years [2–4,12–14]. Though it is clear that solvents can play the role of plasticizer to change glass transition behaviors, a comprehensive investigation on the effect of solvents on glass transition behaviors is still lacking. In this work, we investigate the glass transition behaviors of crosslinked amorphous thermosets with different solvent concentrations through the dynamic mechanical analysis. We aim to understand the effects of solvent concentration on the glass transition region as well as the viscoelastic properties across the glass transition region. The paper is arranged as follows. Section2 presents the methods of material synthesis as well as mechanical characterization. The analysis procedures are also shown in this section. Section3 shows the main results including the effects of solvent on the glass transition region and the relaxation spectrum. Finally, the ﬁndings are summarized in the conclusion part.

2. Experimental Methods 2.1. Material Synthesis The monomer tert-butyl acrylate (tBA), the crosslinker poly (ethylene glycol) dimethacry- late (PEGDMA), with typical molecular weight of 550 and photo-initiator 2,2-dimethoxy-2- phenylacetophenone (DMPA) were ordered from Sigma Aldrich, and isopropyl alcohol (≥99.5%) (IPA) was ordered from Aladdin. All chemicals were used as received. Two different solution were prepared by mixing tBA with PEGDMA, with a weight ratio of 98%:2% or 80%:20%. The photo-initiator, DMPA, was then added to the comonomer solution at a concentration of 0.2 wt.% of the total comonomer weight. The mixture was then injected into two glass slides separated by a 1 mm space and cured in a UV oven (CL-1000L Crosslinker, Analytik Jena, Upland, CA, USA) for 20 min. After the UV curing, the specimens were further thermally cured in an oven at 80 ◦C for 1 h to achieve a full polymerization. Table1 lists the polymers synthesized in this work.

Table 1. The polymer synthesized in this work.

Name of Samples The Mass Ratio (tBA:PEGDMA:DMPA) Acrylate-based polymer with 2 wt.% crosslink density 98:2:0.2 Acrylate-based polymer with 20 wt.% crosslink density 80:20:0.2

As described above, the acrylate-based polymers can be synthesized through one-step photo-polymerization. In addition, the crosslink density, related to the rubbery modulus, and the glass transition region can both be easily tuned in this material system, which makes acrylate-based polymers good candidates as shape-memory polymers [5].

2.2. Treatment by IPA Rectangular specimens with a size of 20 mm × 5 mm × 1 mm were used to measure the swelling ratio. Each specimen was weighed before testing. The specimens were immersed in IPA for different times and then taken out of the solvent and wrapped with aluminum foil for three days at room temperature (20–25 ◦C), then weighed again by a digital balance with an accuracy of 10−4 g. This step is to ensure a homogenous distribution of solvents in specimens. For each measurement, three specimens were repeated. The swelling ratio Sw is then deﬁned as mt − m0 Sw = (1) m0

where m0 is the weight of initial dry specimen and mt is the weight after treated by IPA. Materials 2021, 14, x FOR PEER REVIEW 3 of 13

Materials 2021, 14, 730 3 of 13 where 𝑚 is the weight of initial dry specimen and 𝑚 is the weight after treated by IPA.

2.3. Dynamic Temperature Sweep Tests 2.3. DynamicThe glass Temperature transition Sweep region Tests of polymers was measured using a TA Q800 Dynamic MechanicalThe glass Analyzer transition (DMA, region TA of Instruments, polymers was New measured Castle, DE, using USA). a TA The Q800 specimens Dynamic with Mechanicalor without Analyzertreatment (DMA, in IPA TA were Instruments, subjected to New 0.2% Castle, dynamic DE, USA).strain Thewith specimens a frequency with of 1 orHz without and a treatmentheating rate in IPAof 2 were°C/min. subjected For polymers to 0.2% dynamicwith 2 wt.% strain crosslink with a frequencydensity, the of 1speci- Hz andmens a heating with an rate immersion of 2 ◦C/min. time Forof 30 polymers min, 1, with5 and 2 10 wt.% h were crosslink chosen density, for the the dynamic specimens me- withchanical an immersion characterization, time of 30while min, for 1, 5polymers and 10 h with were 20 chosen wt.% for crosslink the dynamic density, mechanical the speci- characterization,mens with an immersion while for polymerstime of 1, with2 and 20 10 wt.% h were crosslink chosen. density, the specimens with an immersion time of 1, 2 and 10 h were chosen. 2.4. Dynamic Frequency Sweep Tests 2.4. DynamicAmorphous Frequency polymers Sweep Testsexhibit a time-dependent viscoelastic response in the glass transitionAmorphous region, polymers such as stress exhibit relaxation, a time-dependent creep and rate-dependent viscoelastic response stress responses. in the glass The transitiontime scale region, for relaxation such as may stress span relaxation, several creepdecades. and Thus, rate-dependent it is difficult stress to obtain responses. all the Thecharacteristics time scale for of relaxation within may span a single several test. decades. Fortunately, Thus, the it time–temperature is difﬁcult to obtain super- all theposition characteristics (TTS) principal of relaxation works for within many a singleamorph test.ous Fortunately,polymers. Thus, the time–temperatureit is possible to con- superpositionstruct a master (TTS) curve principal for viscoelastic works for manyrespon amorphousses at one polymers.single reference Thus, ittemperature is possible toby constructconducting a master tests at curve various for viscoelastictemperatures. responses at one single reference temperature by conductingHere the tests dynamic at various frequency temperatures. sweep tests are adopted to obtain the relaxation spec- trumHere of polymers the dynamic with frequency different sweepsolvent tests concentrations. are adopted toBefore obtain tests, the Vaseline relaxation was spectrum painted ofon polymers the surface with of differentthe wet specimens solvent concentrations. in order to prevent Before significant tests, Vaseline evaporation was painted of solvents on theat surfacehigh temperatures. of the wet specimens The specimens in order were to preventheated in signiﬁcant a discrete evaporationmanner with of an solvents interval at of high5 °C, temperatures. annealed at each The specimenstest temperature were heatedfor 5 min in aand discrete then mannersubjected with to a an 0.2% interval dynamic of ◦ 5strainC, annealed at 0.3, 1, at 3, each10 and test 30 temperature Hz. for 5 min and then subjected to a 0.2% dynamic strainHere, at 0.3, we 1, 3, use 10 the and results 30 Hz. for dry polymers with 2 wt.% crosslink density as an example Here,to demonstrate we use the the results procedur for dryes to polymers obtain the with master 2 wt.% curv crosslinke and the density relaxation as an spectrum. example toAs demonstrate shown in Figure the procedures 1a, the storage to obtain modulus the masterdepends curve on temperature and the relaxation and frequency. spectrum. The Asfrequency-dependent shown in Figure1a, thestorage storage modulus modulus at dependsvarious temperatures on temperature was and then frequency. shifted to The the frequency-dependent storage modulus at various temperatures was then shifted to the reference temperature 75◦ °C through only a horizontal shift to form a master curve shown referencein Figure temperature 1b. 75 C through only a horizontal shift to form a master curve shown in Figure1b.

(a) (b)

FigureFigure 1. 1.The The storage storage modulus modulus as as a a function function of of frequency frequency for for dry dry polymers polymers with with 2 2 wt.% wt.% crosslink crosslink density density (a ()a measured) measured at at differentdifferent temperatures, temperatures, and and (b ()b shifted) shifted to to form form a mastera master curve. curve.

TheThe shift shift factor factora T𝑎(T(𝑇)) used used to to construct construct the the master master curve curve is is plotted plotted in in Figure Figure2a. 2a. It It cancan also also be be seen seen that that the the shift shift factor factor can can be be ﬁtted fitted by by the the Williams–Landel–Ferry Williams–Landel–Ferry (WLF) (WLF) equationequation [15 [15]] with with the the following following form form

−C0(T − T ) ( ) = 1 0 log a T 0 (2) C2 + T − T0 Materials 2021, 14, x FOR PEER REVIEW 4 of 13

−𝐶 (𝑇 − 𝑇 ) 𝑙𝑜𝑔 𝑎(𝑇) = (2) 𝐶 +𝑇−𝑇 where 𝐶 and 𝐶 are the WLF constants at the reference temperature 𝑇. The reference Materials 2021, 14, 730 temperature 𝑇 is close to the end temperature of the glass transition region. The 4proce- of 13 dures to obtain 𝐶 and 𝐶 can be found in some literatures [15,16]. The WLF constants at 𝑇 , which correspond to the beginning temperature of the glass transition region, can also be 0obtained0 using the following relationship: where C1 and C2 are the WLF constants at the reference temperature T0. The reference temperature T0 is close to the end temperature of the glass transition𝐶 𝐶 region. The procedures 𝐶 =𝐶 +𝑇 −𝑇, 𝐶 = to obtain C0 and C0 can be found in some literatures [15,16]. The WLF constants at Tref(3), 1 2 𝐶 g which correspond to the beginning temperature of the glass transition region, can also be obtainedThe using parameters the following for WLF relationship: constants for polymers with 2 wt.% crosslink density are listed in Table 2, while the corresponding values for polymers with 20 wt.% crosslink den- 0 0 sity are listed in Table 3. Figureg 2b 0plots refthe shift factorsg C at1C the2 glass transition temperature C = C2 + Tg − T0, C = g (3) 𝑇 =40 ℃, with the obtained2 values of 𝐶 and 1𝐶 . C2

100 Experimentally measured shift factor ref o 6 WLF equaiton at T = 40 C 10 WLF equation fitting at T = 75 oC g 0

10-2 104

10-4 2 Shift factor Shift

10 factor Shift

10-6 100

40 50 60 70 80 40 50 60 70 80 o o Temperature ( C) Temperature ( C) (a) (b) 𝑎 (𝑇) FigureFigure 2. 2.The The temperature-dependent temperature-dependent shift shift factor factora T(T) for for dry dry polymers polymers with with 2 wt.%2 wt.% crosslink crosslink density density at at (a ()a the) the reference reference temperature 𝑇 and (b) the glass transition temperatureref 𝑇 . temperature T0 and (b) the glass transition temperature Tg .

Table 2. Parameters of Williams–Landel–FerryThe parameters for WLF(WLF) constants constants for for polymers polymers with with 2 wt.% 2 wt.% crosslink crosslink density. density are Parameter Drylisted in 30 Table min2 , while 1 h the corresponding 5 h 10 h values for polymers Physical with Significance 20 wt.% crosslink den- 𝑇 ℃ sity are listed in Table3. Figure2b plots the shift factors at the glass transition temperature ( ) 75 ref 10 0 −5 −25 g g The reference temperature Tg = 40 °C, with the obtained values of C and C . 𝐶 4.70 5.90 7.78 25.52 24.62 1 2 First WLF constant at 𝑇 𝐶 (℃) 62.34 64.92 79.47 187.80 144.08 Second WLF constant at 𝑇 Table 2. Parameters of Williams–Landel–Ferry (WLF) constants for polymers with 2 wt.% crosslink density. 𝑇 (℃) 40 −25 −45 −70 −80 Glass transition temperature Parameter𝐶 Dry10.72 30 min12.80 1 h17.94 39.03 5 h39.82 10 hFirst WLF Physical constant Signiﬁcance at 𝑇 T (°C) 75 10 0 −5 −25 The reference temperature 0 𝐶 (℃) 27.34 29.92 34.47 122.80 89.08 Second WLF constant at 𝑇 0 C1 4.70 5.90 7.78 25.52 24.62 First WLF constant at T0 0 C2 (°C) 62.34 64.92 79.47 187.80 144.08 Second WLF constant at T0 ref Table 3. Parameters of WLF constants for polymers with 20 wt.% crosslink density. Tg (°C) 40 −25 −45 −70 −80 Glass transition temperature g ref C1Parameter 10.72 Dry 12.80 1 h 17.94 2 h 39.03 10 h 39.82 PhysicalFirst WLF Significance constant at Tg g ref C2 (°C)𝑇 (℃) 27.3460 29.9215 34.4710 122.80−10 89.08 TheSecond reference WLF temperature constant at Tg 𝐶 5.31 10.14 7.81 8.80 First WLF constant at 𝑇 𝐶 (℃) 54.06 79.69 73.84 82.33 Second WLF constant at 𝑇 Table 3. Parameters of WLF constants for polymers with 20 wt.% crosslink density. 𝑇 (℃) 30 −20 −25 −55 Glass transition temperature Parameter𝐶 Dry11.93 18.08 1 h 14.85 2 h19.41 10 hFirst WLF Physical constant Signiﬁcance at 𝑇 T (°C) 60 15 10 −10 The reference temperature 0 𝐶 (℃) 24.06 44.69 38.84 37.33 Second WLF constant at 𝑇 0 C1 5.31 10.14 7.81 8.80 First WLF constant at T0 0 C2 (°C) 54.06 79.69 73.84 82.33 Second WLF constant at T0 ref Tg (°C) 30 −20 −25 −55 Glass transition temperature g ref C1 11.93 18.08 14.85 19.41 First WLF constant at Tg g ref C2 (°C) 24.06 44.69 38.84 37.33 Second WLF constant at Tg

To analyze the effects of solvents on the relaxation behaviors, the master curve of the storage modulus is further ﬁtted by a fractional Zener model [16–18]. The Zener model is composed of a spring in parallel with a Maxwell element. The spring is used to represent the Materials 2021, 14, x FOR PEER REVIEW 5 of 13

To analyze the effects of solvents on the relaxation behaviors, the master curve of the storage modulus is further fitted by a fractional Zener model [16–18]. The Zener model is composed of a spring in parallel with a Maxwell element. The spring is used to represent the equilibrium elastic response while the Maxwell element is used to describe the viscoelastic response. However, it is found that the Maxwell model predicts an exponential Materials 2021, 14, 730 5 of 13 form for the relaxation response, which is not consistent with the actual relaxation response in polymers. Alternatively, the integer order derivative of the Maxwell model can beequilibrium replaced elastic by a response fractional while order the Maxwell derivative, element which is used toprovides describe thea better viscoelastic description on the viscoelasticresponse. However, response it is foundof polymers. that the Maxwell Thus, the model fractional predicts anZener exponential model form in total for contains four parameters:the relaxation the response, rubbery which modulus is not consistent 𝐸, the with glassy the actualmodulus relaxation 𝐸 response, the characteristic in relax- ationpolymers. time Alternatively, τ, and the the fractional integer order order derivative α denoting of the Maxwell the breadth modelcan of bethe replaced relaxation spectrum [17].by a fractionalThe analytical order derivative, expression which of storage provides modulus a better description can be represented on the viscoelastic as, response of polymers. Thus, the fractional Zener model in total contains four parameters: eq neq απ τ the rubbery modulus E , the glassy modulus E𝐸, the( characteristicωτ) + (ωτ relaxation) cos time , and the fractional order α denoting the breadth of the relaxation spectrum [17]. The2 analytical 𝐸(ω) =𝐸 + απ . (4) expression of storage modulus can be represented1+ as,(ωτ) +2(ωτ) cos 2 neq 2α α απ E (ωτ) + (ωτ) cos 2 The rubberyE0 modulus(ω) = Eeq and+ the glassy modulus can be obtained from the modulus in frac 2α α απ . (4) the plateau regions. To determine1 + the(ωτ values) + 2 of(ωτ the) cosparameters2 τ and α, the following function Thewas rubbery minimized: modulus and the glassy modulus can be obtained from the modulus τ α in the plateau regions. To determine the values of the parameters and , the following function was minimized: 𝐸𝑟𝑟𝑜𝑟 = (log 𝐸(ω) −log𝐸 (ω)) (5)

0 2 where 𝐸 (ω) and Error𝐸 (ω)= are∑ logtheE fitted(ω) − logandE fracmeasured(ω) storage modulus,(5) respectively. As0 shown in Figure0 3, the master curve can be well described by the fractional Zener where Efrac(ω) and E (ω) are the ﬁtted and measured storage modulus, respectively. modelAs shownwith the in Figure model3, the parameters master curve listed can be in well Ta describedbles 4 and by the5. However, fractional Zener at the highest frequency,model with the the fractional model parameters Zener model listed inoverestima Tables4 andtes5. the However, storage at themodulus. highest This is because thefrequency, fractional the fractional Zener model Zener model is based overestimates on the eq theuilibrium storage modulus. structure This state. is because At the highest frequency,the fractional corresponding Zener model to is low based temperature, on the equilibrium the polymer structure structure state. At the may highest fall out of the equi- frequency, corresponding to low temperature, the polymer structure may fall out of the libriumequilibrium due due to to structural structural relaxation.relaxation.

FigureFigure 3. 3.Measured Measured and and ﬁtted fitted master master curve for curve dry polymers for dry withpolymers 2 wt.% with crosslink 2 wt.% density. crosslink density.

TableTable 4. 4. ParametersParameters of of the the fractional fractional Zener Zener model model for polymers for polymers with 2 wt.% with crosslink 2 wt.% density. crosslink density.

ParameterParameter Dry Dry 30 min30 min 1 1 h h 5 h 5 h 10 h 10 h Physical Physical Signiﬁcance Significance Eneq (MPa) 872 527 498 170 89 The glassy moduli 𝐸 eq (MPa) 872 527 498 170 89 The glassy moduli E (MPa) 0.58 0.51 0.47 0.20 0.15 The rubbery moduli 𝐸 (MPa)−9 0.58 0.51 0.47 0.20 0.15 The rubbery moduli τ 10 s 2100 2150 80 0.010 0.074 Stress relaxation time at T = T0 τ(10α s) 0.72100 0.68 2150 0.58 80 0.010 0.26 0.074 0.28 Breadth Stress of relaxation relaxation spectrum time at T=𝑇 α 0.7 0.68 0.58 0.26 0.28 Breadth of relaxation spectrum Materials 2021, 14, x FOR PEER REVIEW 6 of 13

Materials 2021, 14, 730 6 of 13 Table 5. Parameters of the fractional Zener model for polymers with 20 wt.% crosslink density.

Parameter Dry 1 h 2 h 10 h Physical Significance Table 5. Parameters of the fractional Zener model for polymers with 20 wt.% crosslink density. 𝐸 (MPa) 1438 805 686 399 The glassy moduli 𝐸Parameter (MPa) Dry 3.53 3.46 1 h 3.43 3.25 2 h 10 h The rubbery Physical moduli Signiﬁcance neq Eτ(10(MPa)s) 14389270 9200 805 1690 68630 399 Stress relaxation The glassytime at moduli T=𝑇 Eeq (MPa)α 3.530.72 0.66 3.46 0.66 0.42 3.43 3.25 Breadth of relaxation The rubbery spectrum moduli −9 τ 10 s 9270 9200 1690 30 Stress relaxation time at T = T0 α 0.72 0.66 0.66 0.42 Breadth of relaxation spectrum 3. Results and Discussions 3. ResultsFigure and 4 plots Discussions the swelling ratio as a function of time. As shown, tBA–co–PEGDMA networksFigure with4 plots 2 wt.% the swelling and 20 ratio wt.% as crosslink a function density of time. Ascan shown, reach tBA–co–PEGDMAan equilibrium swelling statenetworks in around with 210 wt.% h. The and maximum 20 wt.% crosslink swelling density ratio canof acrylate reach an copolymer equilibrium with swelling 2 wt.% and 20state wt.% in crosslink around 10 density h. The are maximum around swelling 200% an ratiod 50% of respectively. acrylate copolymer Polymers with with 2 wt.% a smaller crosslinkand 20 wt.% density crosslink can exhibit density a are larger around swelling 200% andratio, 50% which respectively. is as expected. Polymers In withthermody- namics,a smaller more crosslink elastic density energy can of exhibit stretching a larger polymer swelling chains ratio, is which needed is as for expected. polymers In with a denserthermodynamics, crosslinked more state, elastic which energy results of stretching in a smaller polymer swelling chains ratio. is needed for polymers with a denser crosslinked state, which results in a smaller swelling ratio.

(a) (b) FigureFigure 4. 4.SwellingSwelling ratio ratio of of acrylate polymerpolymer with with (a )(a 2) wt.% 2 wt.% and and (b) 20(b) wt.% 20 wt.% crosslink crosslink density. density.

TheThe storage modulusmodulus of of specimens specimens with with different different immersion immersion time is time shown is inshownFigure in5 . Figure All specimens exhibit a glass transition behavior. For polymers with 2 wt.% crosslink 5. All specimens exhibit a glass transition behavior. For polymers with 2 wt.% crosslink density and immersed in IPA for 10 h, the weight fraction of IPA is around 66%. Thus, the densitymaterial and is inherent immersed a polymeric in IPA for gel. 10 Glass h, the transition weight fraction can still occurof IPA for is thisaround wet polymer66%. Thus, the materialin the extremely is inherent low a temperature polymeric gel. region Glass from transition−70 to − 20can◦ C.still For occur all polymers, for this wet in the polymer inglass the extremely transition region low temperature the modulus region decreases from by − 2–370 to orders −20 °C. as For temperatures all polymers, increases. in the glass transitionWith increasing region the the solvent modulus concentration, decreases the by glass 2–3 transitionorders as regiontemperatures shifts to theincreases. lower With increasingtemperature the region, solvent indicating concentration, an increase the in glas the plasticizations transition region effects. Forshifts both to polymers, the lower tem- ◦ peraturewhen immersed region, inindicating IPA for 10 an h, increase the glass transitionin the plasticization region shifts effects. more than For 80 bothC. Forpolymers, whenpolymers immersed with 2 wt.%in IPA crosslink for 10 density,h, the glass a tremendous transition shift region occurs shifts for specimensmore than with 80 °C. For immersion time of only 30 min in IPA. This indicates that a small amount of solvent can polymers with 2 wt.% crosslink density, a tremendous shift occurs for specimens with induce a pronounced plasticization effect. The glass transition region of polymers with 5 immersionand 10 h immersion time of only time 30 in IPAmin almost in IPA. overlaps. This indicates This is because that a small the plasticization amount of effectsolvent can inducesaturates a pronounced with a large amount plasticization of solvent. effect. In addition The glass to the transition glass transition region region, of polymers both the with 5 andglassy 10 modulush immersion and rubbery time in modulusIPA almost decrease overlaps with. the This increasing is because solvent the concentration.plasticization effect saturates with a large amount of solvent. In addition to the glass transition region, both the glassy modulus and rubbery modulus decrease with the increasing solvent concentration. Materials 2021, 14, x FOR PEER REVIEW 7 of 13

Materials 2021, 14, x FOR PEER REVIEW 7 of 13 Materials 2021, 14, 730 7 of 13

(a) (b)

Figure 5. The storage modulus as a function of temperature of (a) polymers with 2 wt.% crosslinking density and (b) 20 (a) (b) wt.% crosslink density and treatment in isopropyl alcohol (IPA) for different times. FigureFigure 5. The 5. The storage storage modulus modulus as as a a function function ofof temperaturetemperature of of (a )(a polymers) polymers with with 2 wt.% 2 wt.% crosslinking crosslinking density density and (b )and 20 (b) 20 wt.%wt.% crosslink crosslink density density and and treatment treatmentWe then in isopropyl used the alcohol alcohol method (IPA) (IPA) shown for for different different in Section times. times. 2.4 to construct the master curve for various polymer–solvent systems. The results for polymers with 1 and 10 h immersion time We then used the method shown in Section 2.4 to construct the master curve for We then used the method shown in Section 2.4 to construct the master curve for var- invarious IPA are polymer–solvent shown in Figures systems. 6 and The 7 resultsto demonstrate for polymers the with typical 1 and glass 10 h immersiontransitiontime behaviors ofiousin wet IPA polymer–solvent polymers. are shown All in Figures the systems. other6 and cases7 The to demonstrate resultsshow similar for thepolymers behaviors. typical glasswith As transition 1 andshown, 10 behaviors hthe immersion storage of mod- time ulusinwet IPA of polymers. arewet shown polymers All thein Figures otheralso depends cases 6 and show 7on to similar tempdemonstrateerature behaviors. and the As typicalfrequency, shown, glass the storagewhich transition does modulus behaviorsnot show anyofof wet wetqualitative polymers. polymers difference alsoAll the depends other from on cases that temperature ofshow dry similarpo andlymers. frequency, behaviors. A master which As curve shown, does can not the showbe storage constructed, any mod- whichulusqualitative of suggests wet differencepolymers the time–temperature from also that depends of dry polymers.on superposition temperature A master andprinciple curve frequency, can can be constructed,be which applied does whichfor not wet show polymers.anysuggests qualitative The the master time–temperature difference curve of from the superposition storagethat of drymodu po principleluslymers. spans can A a bemasterbroad applied frequencycurve for wetcan polymers.regionbe constructed, for both drywhichThe and master suggests wet curvepolymers, the of time–temperature the indicating storage modulus that superpositiona spans broad a broaddistribution frequency principle of regionrelaxation can be for applied both time dry exists.for and wet polymers.wet polymers, The master indicating curve that of the a broad storage distribution modulus of spans relaxation a broad time frequency exists. region for both dry and wet polymers, indicating that a broad distribution of relaxation time exists.

(a) (b)

Figure 6. Cont. (a) (b) MaterialsMaterials 20212021, 14,,14 x ,FOR 730 PEER REVIEW 8 of 13 8 of 13

Materials 2021, 14, x FOR PEER REVIEW 8 of 13

(c)( c) (d)( d)

FigureFigureFigure 6. The 6. The6. storageThe storage storage modulus modulus modulus forfor for polymerspolymers polymers with with with 2 wt.% 2 2 wt.%wt.% crosslink crosslink density density density and treatmentand and tr eatmenttreatment by IPA by for IPAby ( aIPA )for 1 h (fora and) 1 ( ha (c ))and 1 10 h h, (andc) and 10 (h,c) 10 h, andthe theand master master the master curve curve forcurve for (b) for 1(b h) ( and1b )h 1 and (hd )and 10 (d h.()d 10) 10 h. h.

(a) (b)

FigureFigure 7. 7.The The storage storage modulus modulus for for polymers polymers with with 20 20 wt.% wt.% crosslink crosslink density density and and treatment treatment by by IPA IPA for for (a )(a 1) h1 andh and (c )(c 10) 10 h, h, andand the the master master curve curve for for (b )(b 1) h1 andh and (d )(d 10) 10 h. h. (c) (d) The procedure to construct the master curve also provides the information for the Figure 7. The storage modulus for polymers with 20 wt.% crosslink density and treatment by IPA for (a) 1 h and (c) 10 h, shift factor 𝑎(𝑇). In general, the final value of shift factor of dry polymers is several orders and the master curve for (b) 1 h and (d) 10 h. smaller than that of wet polymers. As shown in Figure 8, all the shift factors can be well The procedure to construct the master curve also provides the information for the shift factor 𝑎(𝑇). In general, the final value of shift factor of dry polymers is several orders smaller than that of wet polymers. As shown in Figure 8, all the shift factors can be well Materials 2021, 14, x FOR PEER REVIEW 9 of 13 Materials 2021, 14, 730 9 of 13

describedThe by procedure the WLF to equation. construct theThe master obtained curve WLF also constants provides the at informationthe reference for temperature the andshift the factor glass atransitionT(T). In general, temperature the ﬁnal for value polyme of shiftrs with factor 2 ofwt.% dry and polymers 20 wt.% is several crosslink den- sityorders are listed smaller in thanTables that 2 of and wet 3 polymers. respective Asly. shown The inWLF Figure equation8, all the can shift be factors derived can based on thebe free well volume described theory by the [19–23], WLF equation. which states The obtained that the WLF free constants volume at of the polymer reference decreases temperature and the glass transition temperature for polymers with 2 wt.% and 20 wt.% withcrosslink a decrease density in are temperature. listed in Tables The2 and WLF3 respectively. equation The has WLF been equation successfully can be derived applied to de- scribebased the on shifting the free volumefactor of theory various [19– 23material], which systems, states that such the free as polymers, volume of polymercomposites and biologicaldecreases materials with a decrease [15,16,24,25]. in temperature. Here The we WLF show equation that even has been for successfullypolymer–solvent applied systems withto describea large fraction the shifting of factorsolvents, of various the WLF material equa systems,tion can such still as provide polymers, a good composites description of theand shift biological factors. materials Thus, the [15, 16fundamental,24,25]. Here wemechan show thatism even of glass for polymer–solvent transition is the systems same for the wetwith polymers a large fractionand dry of polymers. solvents, the As WLF shown equation in Tables can still 2 and provide 3, both a good 𝑇 descriptionand 𝑇 decrease of the shift factors. Thus, the fundamental mechanism of glass transition is the same with immersion time, indicating that the glass transition region is shifted to lowerref tem- for the wet polymers and dry polymers. As shown in Tables2 and3, both T0 and Tg peraturedecrease regions. with immersion The parameter time, indicating 𝐶 is that larger the glass for transitionwet polymers region isthan shifted the to dry lower polymers. g Thistemperature indicates regions.that the The Kauzmann parameter temperatureC2 is larger for is wet a further polymers departure than the dry from polymers. the onset glass transitionThis indicates temperature. that the Kauzmann temperature is a further departure from the onset glass transition temperature.

(a) (b) FigureFigure 8. The 8. temperature-dependentThe temperature-dependent shift shift factor factor forfor polymers polymers with with (a) 2(a wt.%) 2 wt.% and (andb) 20 ( wt.%b) 20 crosslink wt.% crosslink density. density.

We further analyzed the master curve using the fractional Zener model. Figures9 and 10 We further analyzed the master curve using the fractional Zener model. Figures 9 plot the master curves measured from experiments together with the ﬁtted results for both andpolymers. 10 plot Thethe obtainedmaster curves model parameters measured and from their experiments signiﬁcance are together listed in Tableswith the4 and fitted5 . results forAs both shown, polymers. the fractional The obtained Zener model model with parameters four parameters and their can generally significance describe are thelisted in Ta- blesmaster 4 and curve 5. As obtained shown, through the fractional the time–temperature Zener model superposition with four method.parameters Similar can to generally describethe results the ofmaster the dry curve polymers, obtained it is also through found thatthe thetime–temperature model fails to capture superposition the storage method. Similarmodulus to the in the results high-frequency of the dry region. polymers, This is probablyit is also because found this that region the model corresponds fails toto capture the results measured close to the glass transition temperature. The polymer structures may thefail storage out of themodulus equilibrium in the state high-frequency due to structural relaxation.region. This In this is probably situation, the because measured this region correspondsstorage modulus to the is results smaller thanmeasured that in theclose equilibrium to the glass state. transition temperature. The polymer structures may fail out of the equilibrium state due to structural relaxation. In this situation, the measured storage modulus is smaller than that in the equilibrium state. MaterialsMaterials2021 2021, 14, ,14 730, x FOR PEER REVIEW 1010 of 13of 13

(a) (b) Materials 2021, 14, x FOR PEER REVIEW 11 of 13 FigureFigure 9. Measured9. Measured and and ﬁtted fitted master master curve curve for for polymers polymers with wi 2th wt.% 2 wt.% crosslink crosslink density density and and subjected subjected to (a to) 1 (a h) and1 h and (b) 10 (b h) 10 immersionh immersion time time in IPA. in IPA.

The parameters listed in Tables 4 and 5 can directly reflect the influence of solvents on the glass transition behaviors of polymers. First, both the glassy modulus and rubbery modulus decrease with increasing solvent concentration. The glassy modulus for polymers with 2 wt.% crosslink density decreases by almost 10 times from dry state to fully saturated state. In comparison, the rubbery modulus only changes from 0.58 to 0.15 MPa. For polymers with 20 wt.% crosslink density, the rubbery modulus only slightly changes with solvent. For polymer gels, it has been shown that the rubbery modulus in the wet / state is scaled with that of dry state as 𝐸 =𝐸 /𝐽 , where 𝐽 is the ratio of volume after swelling and that of the initial dry state [26]. The two acrylate-based polymers have a density of around 1.03–1.05 g/cm3, while IPA has a density of 0.785 g/cm3. Based on the swelling results shown in Figure 4, for acrylate-based polymer with 2 wt.% crosslink density the ratio of the volume in the equilibrium swelling state and the dry state is around 3.7, while that for polymers with 20 wt.% crosslink density is around 1.7. The value of J and the corresponding density of other wet polymers can also be calculated based on the

assumption(a) of the volumetric incompressibility for mixture.(b) Thus, the measured value is generally consistent with the above relationship. There are few theories to relate the glassy modulus of the wet polymers with dry polymers. Our results can potentially be used to validate a future developed theory. The value for the relaxation breadth α continuously decreases with increasing solvent concentration for both polymers. A smaller value of 𝛼 indicates a broader relaxation spectrum. Thus, solvents can expand the breadth of the relaxation spectrum. So far, few works have been performed to investigate the effects of solvent on the relaxation spectrum both in experimental and theoretical aspects. One possible explanation is that extra chemical or physical bonding may be formed between solvent and polymer molecules. The strength of intermolecular interaction in polymers can be further affected by the newly formed bonding, which further results in a change in the relaxation responses [27]. In addition to solvents, some works [28,29] also show that the relaxation spectrum of dry polymers can be changed by mechanical deformation.

FigureFigure 10. 10.Measured Measured and and ﬁtted fitted master master curve curve for for polymers polymers with with 20 20 wt.% wt.% crosslink crosslink density density and and subjected subjected to to ( a()a) 0 0 h, h, ( b(b)) 1 1 h, h, (c()c 2) 2 h h and and (d ()d 10) 10 h h immersion immersion time time in in IPA. IPA.

4. Conclusions Glass transition can be affected by many factors, such as thermal treatment and plas- tination effects. In the past several years, our group has combined experiments and theory to comprehensively investigate the effects of physical aging on the glass transition behaviors in amorphous polymers [30,31]. In this work, we focus on understanding the effects of solvents on glass transition. A series of experiments were carried out to characterize the glass transition of amorphous polymers with different solvent concentrations. It is shown that all polymer–solvent systems investigated exhibit a glass transition behavior, even for the systems with more than 60 wt.% of solvents. The glass transition region shifts to a lower temperature with increasing solvent concentration. The classic methods to analyze the dry polymers, such as the time–temperature superposition principal, can still be applied for wet polymers. Specifically, master curves of storage modulus as a function of frequency can be constructed using the time–temperature superposition principal. The shift factors used to construct the master curves can be well fitted by the WLF equation, which indicates the underlying mechanism for glass transition is the same for wet polymers and dry polymers. An analytical model with four parameters was also used to fit the master curve. It is found that solvents can decrease the rubbery modulus and the glassy modulus. More importantly, the presence of solvents can significantly expand the breadth of the relaxation spectrum, which may be attributed to the change of strength of intermolecular interaction caused by the chemical or physical bonding newly formed between Materials 2021, 14, 730 11 of 13

The parameters listed in Tables4 and5 can directly reﬂect the inﬂuence of solvents on the glass transition behaviors of polymers. First, both the glassy modulus and rubbery modulus decrease with increasing solvent concentration. The glassy modulus for polymers with 2 wt.% crosslink density decreases by almost 10 times from dry state to fully saturated state. In comparison, the rubbery modulus only changes from 0.58 to 0.15 MPa. For polymers with 20 wt.% crosslink density, the rubbery modulus only slightly changes with solvent. For polymer gels, it has been shown that the rubbery modulus in the wet state eq eq 1/3 is scaled with that of dry state as Ew = Ed /J , where J is the ratio of volume after swelling and that of the initial dry state [26]. The two acrylate-based polymers have a density of around 1.03–1.05 g/cm3, while IPA has a density of 0.785 g/cm3. Based on the swelling results shown in Figure4, for acrylate-based polymer with 2 wt.% crosslink density the ratio of the volume in the equilibrium swelling state and the dry state is around 3.7, while that for polymers with 20 wt.% crosslink density is around 1.7. The value of J and the corresponding density of other wet polymers can also be calculated based on the assumption of the volumetric incompressibility for mixture. Thus, the measured value is generally consistent with the above relationship. There are few theories to relate the glassy modulus of the wet polymers with dry polymers. Our results can potentially be used to validate a future developed theory. The value for the relaxation breadth α continuously decreases with increasing solvent concentration for both polymers. A smaller value of α indicates a broader relaxation spectrum. Thus, solvents can expand the breadth of the relaxation spectrum. So far, few works have been performed to investigate the effects of solvent on the relaxation spectrum both in experimental and theoretical aspects. One possible explanation is that extra chemical or physical bonding may be formed between solvent and polymer molecules. The strength of intermolecular interaction in polymers can be further affected by the newly formed bonding, which further results in a change in the relaxation responses [27]. In addition to solvents, some works [28,29] also show that the relaxation spectrum of dry polymers can be changed by mechanical deformation.

4. Conclusions Glass transition can be affected by many factors, such as thermal treatment and plasti- nation effects. In the past several years, our group has combined experiments and theory to comprehensively investigate the effects of physical aging on the glass transition behaviors in amorphous polymers [30,31]. In this work, we focus on understanding the effects of solvents on glass transition. A series of experiments were carried out to characterize the glass transition of amorphous polymers with different solvent concentrations. It is shown that all polymer–solvent systems investigated exhibit a glass transition behavior, even for the systems with more than 60 wt.% of solvents. The glass transition region shifts to a lower temperature with increasing solvent concentration. The classic methods to analyze the dry polymers, such as the time–temperature superposition principal, can still be applied for wet polymers. Specifically, master curves of storage modulus as a function of frequency can be constructed using the time–temperature superposition principal. The shift factors used to construct the master curves can be well fitted by the WLF equation, which indicates the underlying mechanism for glass transition is the same for wet polymers and dry polymers. An analytical model with four parameters was also used to fit the master curve. It is found that solvents can decrease the rubbery modulus and the glassy modulus. More importantly, the presence of solvents can significantly expand the breadth of the relaxation spectrum, which may be attributed to the change of strength of intermolecular interaction caused by the chemical or physical bonding newly formed between solvent and polymer molecules [27]. This raises more challenges for developing theories for the glass transition behaviors of polymers and polymer–solvent systems [32,33].

Author Contributions: Conceptualization, R.X.; methodology, software, investigation, writing–- original draft preparation, H.L. and R.X.; writing—review and editing, supervision, funding acquisi- tion, R.X. All authors have read and agreed to the published version of the manuscript. Materials 2021, 14, 730 12 of 13

Funding: This work is supported by the National Natural Science Foundation of China (Grant No. 12022204, 11872170). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Luis de la Fuente, J. An analysis of the thermal aging behaviour in high-performance energetic composites through the glass transition temperature. Polym. Degrad. Stabil. 2009, 94, 664–669. [CrossRef] 2. Guo, Y.F.; Lv, Z.Y.; Huo, Y.R.; Sun, L.J.; Chen, S.; Liu, Z.H.; He, C.L.; Bi, X.P.; Fan, X.Q.; You, Z.W. A biodegradable functional water-responsive shape memory polymer for biomedical applications. J. Mat. Chem. B 2019, 7, 123–132. [CrossRef] 3. Yang, B.; Huang, W.M.; Li, C.; Lee, C.M.; Li, L. On the effects of moisture in a polyurethane shape memory polymer. Smart Mater. Struct. 2004, 13, 191–195. [CrossRef] 4. Huang, W.M.; Yang, B.; An, L.; Li, C.; Chan, Y.S. Water-driven programmable polyurethane shape memory polymer: Demonstra- tion and mechanism. Appl. Phys. Lett. 2005, 86, 114105. [CrossRef] 5. Xiao, R.; Zhang, C.; Gou, X.; Huang, W.M. Tunable shape-memory behaviors in amorphous polymers through bound solvent. Mater. Lett. 2017, 209, 131–133. [CrossRef] 6. Zhang, H.; Düring, L.; Kovacs, G.; Yuan, W.; Niu, X.; Pei, Q. Interpenetrating polymer networks based on acrylic elastomers and plasticizers with improved actuation temperature range. Polym. Int. 2010, 59, 384–390. [CrossRef] 7. Simon, P.P.; Ploehn, H.J. Modeling the effect of plasticizer on the viscoelastic response of crosslinked polymers using the tube-junction model. J. Rheol. 2000, 44, 169–183. [CrossRef] 8. Dimarzio, E.A.; Gibbs, H.J. Molecular interpretation of glass temperature depression by plasticizers. J. Polym. Sci. 1963, 1, 1417–1428. [CrossRef] 9. Vieira, M.G.A.; da Silva, M.A.; dos Santos, L.O.; Beppu, M.M. Natural-based plasticizers and biopolymer films: A review. Eur. Polym. J. 2011, 47, 254–263. [CrossRef] 10. Da Silva, M.A.; Vieira, M.G.A.; Macumoto, A.C.G.; Beppu, M.M. Polyvinylchloride (PVC) and natural rubber films plasticized with a natural polymeric plasticizer obtained through polyesterification of rice fatty acid. Polym. Test. 2011, 30, 478–484. [CrossRef] 11. Kawai, K.; Hagura, Y. Discontinuous and heterogeneous glass transition behavior of carbohydrate polymer-plasticizer systems. Carbohydr. Polym. 2012, 89, 836–841. [CrossRef][PubMed] 12. Xiao, R.; Guo, J.; Nguyen, T.D. Modeling the multiple shape memory effect and temperature memory effect in amorphous polymers. RSC Adv. 2015, 5, 416–423. [CrossRef] 13. Lv, H.; Leng, J.; Liu, Y.; Du, S. Shape-memory polymer in response to solution. Adv. Eng. Mater. 2008, 10, 592–595. [CrossRef] 14. Boyle, A.J.; Weems, A.C.; Hasan, S.M.; Nash, L.D.; Monroe, M.B.B.; Maitland, D.J. Solvent stimulated actuation of polyurethane- based shape memory polymer foams using dimethyl sulfoxide and ethanol. Smart Mater. Struct. 2016, 25, 075014. [CrossRef] [PubMed] 15. Ferry, J.D. Viscoelastic Properties of Polymers; John Wiley and Sons: New York, NY, USA, 1980. 16. Xiao, R.; Choi, J.; Lakhera, N.; Yakacki, C.M.; Frick, C.P.; Nguyen, T.D. Modeling the glass transition of amorphous networks for shape-memory behavior. J. Mech. Phys. Solids 2013, 61, 1612–1635. [CrossRef] 17. Xiao, R.; Sun, H.G.; Chen, W. An equivalence between generalized Maxwell model and fractional Zener model. Mech. Mater. 2016, 100, 148–153. [CrossRef] 18. Haupt, P.; Lion, A.; Backhaus, E. On the dynamic behaviour of polymers under finite strains: Constitutive modelling and identification of parameters. Int. J. Solids Struct. 2000, 37, 3633–3646. [CrossRef] 19. Zielinski, J.M.; Duda, J.L. Predicting polymer solvent diffusion-coefficients using free-volume theory. AIChE J. 1992, 38, 405–415. [CrossRef] 20. Vrentas, J.S.; Duda, J.L. A free-volume interpretation of influence of glass-transition on diffusion in amorphous polymers. J. Appl. Polym. Sci. 1978, 22, 2325–2339. [CrossRef] 21. Yildiz, M.E.; Kokini, J.L. Determination of Williams-Landel-Ferry constants for a food polymer system: Effect of water activity and moisture content. J. Rheol. 2001, 45, 903–912. [CrossRef] 22. Kelley, F.N.; Bueche, F. Viscosity and glass temperature relations for polymer-diluent systems. J. Polym. Sci. 1961, 50, 549–556. [CrossRef] 23. Losi, G.U.; Knauss, W.G. Free volume theory and nonlinear thermoviscoelasticity. Polym. Eng. Sci. 1992, 32, 542–557. [CrossRef] 24. Chang, F.C.; Lam, F.; Kadla, J.F. Application of time–temperature–stress superposition on creep of wood–plastic composites. Mech. Time Depend. Mater. 2013, 17, 427–437. [CrossRef] 25. Maidannyk, V.A.; Roos, Y.H. Modification of the WLF model for characterization of the relaxation time-temperature relationship in trehalose-whey protein isolate systems. J. Food Eng. 2016, 188, 21–31. [CrossRef] Materials 2021, 14, 730 13 of 13

26. Okumura, D.; Kawabata, H.; Chester, S.A. A general expression for linearized properties of swollen elastomers undergoing large deformations. J. Mech. Phys. Solids 2020, 135, 103805. [CrossRef] 27. Wu, C.; Wei, C.; Guo, W.; Wu, C. Dynamic mechanical properties of acrylic rubber blended with phenolic resin. J. Appl. Polym. Sci. 2008, 109, 2065–2070. [CrossRef] 28. Lee, H.N.; Riggleman, R.A.; de Pablo, J.J.; Ediger, M.D. Deformation-induced mobility in polymer glasses during multistep creep experiments and simulations. Macromolecules 2009, 42, 4328–4336. [CrossRef] 29. Dai, L.; Tian, C.; Xiao, R. Modeling the thermo-mechanical behavior and constrained recovery performance of cold-programmed amorphous shape-memory polymers. Int. J. Plast. 2020, 127, 102654. [CrossRef] 30. Guo, J.; Li, Z.; Long, J.; Xiao, R. Modeling the effect of physical aging on the stress response of amorphous polymers based on a two-temperature continuum theory. Mech. Mater. 2020, 143, 103335. [CrossRef] 31. Shi, Q.; Xiao, R.; Yang, H.; Lei, D. Effects of physical aging on thermomechanical behaviors of poly (ethylene terephthalate)-glycol (PETG). Polym. Plast. Technol. Mater. 2020, 59, 835–846. [CrossRef] 32. Lu, H.B.; Wang, X.D.; Hossain, M.; Fu, Y.Q. A Methodology of Hydrodynamic Complexity in Topologically Hyper-Branched Polymers Undergoing Hierarchical Multiple Relaxations. Macromol. Chem. Phys. 2020, 15, 2000052. [CrossRef] 33. Liao, Z.S.; Hossain, M.; Yao, X.H.; Navaratne, R.; Chagnon, G. A comprehensive thermo-viscoelastic experimental investigation of Ecoﬂex polymer. Polym. Test. 2020, 86, 106478. [CrossRef]