NMR and DSC study of temperature-induced phase transition in aqueous solutions of poly(N-isopropylmethacrylamide-co-acrylamide) copolymers

Combination of ¹H NMR spectroscopy and differential scanning calorimetry (DSC) was used to investigate temperature-induced phase transition in D₂O solutions of poly(N-isopropylmethacrylamide-co-acrylamide) random copolymers. Both the NMR and DSC data showed dependence on the acrylamide (AAm) content in the copolymer; with increasing AAm content, the phase transition is shifted to higher temperatures, and both phase-separated fractions determined by NMR and change of the enthalpy determined by DSC decrease faster than the content of thermosensitive N-isopropylmethacrylamide (NIPMAm) units in the copolymer. NMR data were used to construct van't Hoff plots, and changes of the enthalpy ΔH and entropy ΔS, characterizing the phase transition, were determined. As it follows from comparison of NMR and DSC thermodynamical parameters (ΔH values), the size of the cooperative units (domains), undergoing the transition as a whole, decreases with increasing AAm content in the copolymer since the NIPMAm collapsed domains are separated by regions with hydrated AAm and surrounding NIPMAm sequences.     

Synthesis, characterization and thermoreversible behaviours of poly(dimethyl siloxane)/poly(N-isopropyl acrylamide) semi-interpenetrating networks

Simultaneous and sequential poly(N-isopropyl acrylamide) (PNIPAAm)/poly(dimethyl siloxane) (PDMS) semi-interpenetrating polymer networks (IPNs) with different linear PDMS contents were prepared by free radical polymerization method. Their phase morphologies have been characterized by FTIR, DSC and SEM. The simultaneous semi-IPNs exhibited phase transition temperatures (T_pt) shifted higher temperature from glass transition temperatures (T_g) of their respective homopolymers, suggesting a heterophase morphology and only physical entanglement between the PNIPAAm network and linear PDMS with high molecular weight (Mn≈9000 g/mol). For sequential semi-IPNs, the shift of T_pts towards lower temperature suggested that the chemical interaction between the constituents of the IPNs increased with increasing PDMS content in the network. In addition, these semi-IPNs were characterized for their thermo-sensitive behaviour by equilibrium swelling studies. The results showed that incorporation of hydrophobic PDMS polymer into the thermo- and pH-sensitive PNIPAAm and P(NIPAAm-co-IA) (itaconic acid) hydrogels by semi-IPN formation decreased swelling degrees of IPNs without affecting their LCSTs whereas addition of acrylated PDMS (Tegomer V-Si 2250) as crosslinker instead of N,N^′-methylenebisacrylamide (BIS) into the structures of these hydrogels changed their LCSTs along with their swelling degrees.     

Preparation and characterization of poly(N-isopropylacrylamide)-modified poly(2-hydroxyethyl acrylate) hydrogels by interpenetrating polymer networks for sustained drug release

In order to investigate the influence of hydrophobic moieties formed by poly(N-isopropylacrylamide) (PNIPAm) in a hydrogel matrix on the release behavior of the hydrogel, a series of poly(N-isopropylacrylamide) (PNIPAm)-modified poly(2-hydroxyethyl acrylate-co-2-hydroxyethyl 2-hydroxyethyl methacrylate) (P(HEA-co-HEMA)) via interpenetrating polymer networks (IPNs) were prepared by a sequential UV solution polymerization. Interestingly, it was found that P(HEA-co-HEMA)/PNIPAm IPN indicated a single glass transition temperature (T(g)) and the T(g)s of the IPNs increased with an increase in the PNIPAm component. This phenomenon may be attributed to hydrogen bonding between the two polymer networks, but the hydrogen bonding exerts less influence on the swelling behavior of the IPNs, due to the fact that IPNs can respond to changes in temperature like PNIPAm. Using 2-[(diphenylmethyl)sulphiny]acetamide (modafinil, MOD) and p-hydroxybenzoic acid (HBA) as model drug compounds, the release behavior of the IPNs was studied at body temperature, and it was found that the presence of PNIPAm could retard drug release regardless of the solubility of the drugs. Release profiles of HBA from the IPNs and their component samples as a function of time at 37 degrees C.     

New interpenetrating polymer networks based on uralkyd/poly(glycidyl methacrylate)

Semi- and full-interpenetrating polymer networks (IPNs) based on uralkyd resin (UA)/poly(glycidyl methacrylate) were synthesized in the laboratory by the sequential technique. Infrared spectra of the homopolymers and the IPNs were studied. A study of the mechanical properties viz. tensile strength and elongation percentage was carried out. The apparent densities of the homopolymers and their IPNs were determined and compared. Glass transition studies of the IPNs were conducted with the aid of differential scanning calorimetry (DSC). Phase morphology of the IPNs was observed using scanning electron microscopy (SEM). DSC results revealed a single glass transition temperature (T_g) for both the semi- as well as the full-IPNs suggesting good interpenetration in them. The SEM micrographs as well as the IR-spectra gave an indication that apart form the interpenetration phenomena, grafting reaction between the -NCO groups of UA and the epoxy group of glycidyl methacrylate might have occurred to some extent.     

Heterogeneity of glass transition dynamics in polyurethane‐poly(2‐hydroxyethyl methacrylate) semi‐interpenetrating polymer networks

The peculiarities of segmental dynamics over the temperature range of ?140 to 180 °C were studied in polyurethane‐poly(2‐hydroxyethyl methacrylate) semi‐interpenetrating polymer networks (PU‐PHEMA semi‐IPNs) with two‐phase, nanoheterogeneous structure. The networks were synthesized by the sequential method when the PU network was obtained from poly(oxypropylene glycol) (PPG) and adduct of trimethylolpropane (TMP) and toluylene diisocyanate (TDI), and then swollen with 2‐hydroxyethyl methacrylate monomer with its subsequent photopolymerization. PHEMA content in the semi‐IPNs varied from 10 to 57 wt %. Laser‐interferometric creep rate spectroscopy (CRS), supplemented with differential scanning calorimetry (DSC), was used for discrete dynamic analysis of these IPNs. The effects of anomalous, large broadening of the PHEMA glass transition to higher temperatures in comparison with that of neat PHEMA, despite much lower T_g of the PU constituent, and the pronounced heterogeneity of glass transition dynamics were found in these networks. Up to 3 or 4 overlapping creep rate peaks, characterizing different segmental dynamics modes, have been registered within both PU and PHEMA glass transitions in these semi‐IPNs. On the whole, the united semi‐IPN glass transition ranged virtually from ?60 to 160 °C. As proved by IR spectra, some hybridization of the semi‐IPN constituents took place, and therefore the effects observed could be properly interpreted in the framework of the notion of “constrained dynamics.” The peculiar segmental dynamics in the semi‐IPNs studied may help in developing advanced biomedical, damping, and membrane materials based thereon. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 963–975, 2007     

Poly (2,6-dimethyl-1,4-phenylene oxide)/poly (butadiene) diol interpenetrating polymer networks: Phase morphological characteristics and glass transition temperature behavior

The fracture surface topography of simultaneous interpenetrating polymer networks (SIN'S) of poly (2,6-dimethyl-1,4 phenylene oxide) (PPO)/poly (butadiene) diol was examined by scanning electron microscopy (SEM). The phase morphology of the IPNs is highly dependent on PPO/poly (butadiene) diol mass ratio. A single-phase morphology of the IPNs was achieved when the content of poly (butadiene) diol is below 40 wt % in the initial composition. The SEM observations are also wholly consistent with the glass transition temperature behavior obtained in the differential scanning calorimetric (DSC) measurements. © 1994 John Wiley & Sons, Inc.     

Characterization and degradation of poly(methylphenylsiloxane)–poly(methyl methacrylate) interpenetrating polymer networks

Poly(methylphenylsiloxane)–poly(methyl methacrylate) interpenetrating polymer networks (PMPS–PMMA IPNs) were prepared by in situ sequential condensation of poly(methylphenylsiloxane) with tetramethyl orthosilicate and polymerization of methyl methacrylate. PMPS–PMMA IPNs were characterized by infrared (IR), differential scanning calorimetry (DSC), and ²⁹Si and ¹³C nuclear magnetic resonance (NMR). The mobility of PMPS segments in IPNs, investigated by proton spin–spin relaxation T₂ measurements, is seriously restricted. The PMPS networks have influence on the average activation energy E_a,av of MMA segments in thermal degradation at initial conversion. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1717–1724, 1999     

Phase transition in swollen gels 31. Swelling and mechanical behaviour of interpenetrating networks composed of poly(1-vinyl-2-pyrrolidone) and polyacrylamide in water/acetone mixtures

Swelling and mechanical behaviour of interpenetrating positively charged polymer networks (IPNs), composed of poly(1-vinyl-2-pyrrolidone) (PVP) networks and polyacrylamide (PAAm) networks, was investigated in water/acetone mixtures. The first PVP networks were prepared by radiation polymerization at room temperature; after that the PVP networks were swollen in PAAm aqueous solutions and the networks were prepared by thermal copolymerization at 65 °C. The IPNs were prepared with various amounts of the two charged comonomers (quaternary ammonium salts) in the presence of crosslinkers. Two transition regions, detected in the dependence of swelling ratio X on acetone concentration a, suggest that a two-phase structure was formed. The first transition, located between 44 and 60 vol% of acetone, corresponds to PAAm networks, while the second transition, located at 75 vol% of acetone, corresponds to PVP networks. Depending on the amount of positive charges bound to chains, both transitions exhibit continuous or discontinuous character; this fact indicates that intermolecular interactions between the two components occur with the formation of IPNs (e.g., more polar, charged PVP component increases the extent of hydrogen bonding and makes acetone less effective solvent for IPNs at the PAAm transition). The dependences of log G on log X are roughly the same regardless of charge concentrations; this means that the mechanical behaviour is predominantly determined by the degree of swelling for all gels.     

Thermoresponsive behavior of poly(n-isopropylacrylamide) hydrogels containing gold nanostructures

We report the changes in the structure and thermoresponsive behavior of poly(N-isopropylacrylamide) (PNIPAm) hydrogels when gold nanostructures are synthesized in situ within the hydrogel matrix. Cross-linked PNIPAm hydrogels were synthesized using NIPAm and 0.00-3.50% (w/w versus NIPAm) of N,N'-methylenebisacrylamide (MBAm) and/or N,N'-cystaminebisacrylamide (CBAm) as cross-linking agents. The hydrogels were soaked in potassium tetrachloroaurate to introduce gold ions. The hydrogels containing Au3+ were then immersed in a sodium borohydride solution to reduce the gold ions. Infrared spectroscopy, UV-visible spectroscopy, and equilibrium swelling were used to examine the structural/physical differences between gels of different compositions; UV-visible spectroscopy and mass measurements were used to observe the kinetics and thermodynamics of the hydrogel volume phase transition. These studies revealed several differences in the physical characteristics and thermoresponsive behavior of hydrogels based on cross-linker identity and the presence or absence of gold nanostructures. Hydrogels with gold nanostructures and high CBAm and low MBAm content have equilibrium swelling masses 3-20 times their native analogues. In comparison, gold-containing hydrogels with high MBAm and low CBAm content have swelling masses that are equal to their native analogues. Additionally, the gold-containing PNIPAm hydrogels cross-linked with only CBAm have a deswelling temperature of approximately 40 degrees C, approximately 8 degrees C above the samples cross-linked with only MBAm. Varying the CBAm content and introducing gold enables tuning of the deswelling temperature.     

Electrically conductive pseudo IPNs of linear poly(carbonate-urethane) and crosslinked polychloroprene

A number of pseudo IPNs of poly(carbonate-urethane) (PCU) and crosslinked polychloroprene (CR) have been synthesized and characterized. The single phase morphology of these novel pseudo IPNs has been confirmed by DSC measurements and electron microscopy. The room temperature electrical conductives of homopolymers and pseudo IPNs have been investigated for both undoped and iodine-doped samples. © 1994 John Wiley & Sons, Inc.     

Tailoring thermoresponsive nanostructured poly(N-isopropylacrylamide) hydrogels made with poly(acrylamide) nanoparticles

The thermoresponsive behavior and mechanical properties of nanostructured hydrogels, which consist of poly(acrylamide) nanoparticles embedded in a cross-linked poly(N-isopropylacrylamide) hydrogel matrix, are reported here. Nanostructured hydrogels exhibit a tuned volume phase transition temperature (T _VPT), which varies with nanoparticle content in the range from 32 up to 39–40 °C. Moreover, larger equilibrium water uptake, faster swelling and de-swelling rates, and larger equilibrium swelling at 25 °C were obtained with nanostructured hydrogels compared with those of conventional ones. Elastic and Young’s moduli were larger than those of conventional hydrogels at similar swelling ratios. The tuned T _VPT and the de-swelling rate were predicted with a modified Flory–Rehner equation coupled with a mixing rule that considers the contribution of both polymers. These behaviors are explained by a combination of hydrophilic/hydrophobic interactions and by the controlled inhomogeneities (nanoparticles) introduced by the method of synthesis.     

Synthesis and characterization of semi- and full-interpenetrating polymer networks of poly(2,6-dimethyl-1,4-phenylene oxide) and polydimethylsiloxane

The synthesis and characterization of pseudo or semi- and full-interpenetrating polymer networks (IPNs) of poly(2,6-dimethyl-1,4-phenylene oxide) and polydimethylsiloxane were performed. We observed that in full IPNs, the elasticity of the IPN samples increased very drastically, as the composition of polydimethylsiloxane increased (i.e. 0–60%) while the tensile strength (TS) and the glass transition temperature (T_g) decreases. The pseudo IPNs appeared to consist of two phases while the full IPNs of lower siloxane content were miscible.     

Glass transition study of blends of poly(N-methyldodecano-12-lactam) with poly(styrene-co-acrylic acid)

The blends of poly(N-methyldodecano-12-lactam) (MPA) with poly(styrene-co-acrylic acid) (PSAA) prepared from dioxane solutions were studied by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The experimental DSC data of glass transition temperature T_g as a function of composition of amorphous phase were fitted for the as-prepared and re-scanned samples using theoretical approaches. The as-prepared blends show monotonic single-T_g dependence. The values of the Gordon-Taylor coefficient not far from unity suggest miscibility of the blend system in amorphous phase in the whole concentration range. As documented by FTIR, this miscibility is associated with hydrogen bonds between COOH groups of the acrylic acid units in PSAA molecules acting as the H-bond donor and CO groups of MPA acting as the H-bond acceptor. The T_g-dependencies obtained form the second runs have a profound sigmoid character. The Schneider treatment induced an idea of partial limited miscibility in the MPA/PSAA blends caused by prevalence of homogeneous contacts. The difference in T_g between the first and second run can partly be attributed to higher crystallinities in the former.     

Synthesis and characterization of interpenetrating polymer networks with nonlinear optical properties

We report the synthesis and characterization of interpenetrating polymer networks (IPNs) exhibiting nonlinear optical (NLO) properties. The network consists of aliphatic polycarbonate urethane (PCU) and poly(methyl methacrylate-co-N,N-disubstituted urea), with a nonlinear optical (NLO) chromophore incorporated into N,N-disubstituted urea. The full IPNs have only one T_g, as determined by differential scanning calorimetry (DSC), together with scanning electron microscopy (SEM) observations, suggest a single phase morphology. The thin films of IPNs are transparent and the unpoled samples produced second harmonic generation (SHG) signals at room temperature. This result indicates that the NLO chromophore is oriented noncentrosymmetrically during the IPN formation process and is tightly held between the permanent entanglements of the two component networks of the IPN. © 1996 John Wiley & Sons, Inc.     

Preparation and characterization of poly (N-isopropylacrylamide)/polyvinylamine core-shell microgels

In this paper, well-defined temperature- and pH-sensitive core-shell microgels were synthesized by graft copolymerization in the absence of surfactant and stabilizer. The microgel particles consisted of poly (N-isopropylacrylamide (NIPAm)) core crosslinked with N, N′-methylene-bisacrylamide (MBA) and polyvinylamine (PVAm) shell. The effect of MBA content and NIPAm/PVAm ratio on microgel size was investigated. SEM showed that the microgels were spherical and had narrow particle-size distribution. TEM images of the microgels clearly displayed well-defined core-shell morphologies. Zeta-potential measurement further elucidated that the microgels possessed positively charged PVAm molecules on the microgel surface. Turbidity measurement and ¹H-nuclear magnetic resonance (NMR) experiments indicated that the VPTT of microgels was the same as the LCST of PNIPAm. ¹H-NMR experiments also inferred that the methyl proton of N-isopropylacrylamide appeared three peaks and responded to hydrogen-bonding interaction including polymer chain with water molecular, intramolecular interaction and intermolecular interaction, respectively.     

Synthesis of hyperbranched poly(ester urethane isocyanates) and their derivatives

Hyperbranched poly(ester urethanes) containing end isocyanate groups have been synthesized via the reaction of hyperbranched aliphatic polyesterpolydiols of three generations with tolylene diisocyanate. The interaction of these compounds with N,N-dimethylaminoethanol yields their functional derivatives. The polymers are characterized by functional analysis, ¹H NMR and IR spectroscopy, and DSC. It has been demonstrated that the incorporation of urethane moieties leads to development of the microheterogeneous structure of hyperbranched polymers.     

Study on poly(oxybenzoate-p-trimethylene terephthalate) copolymer

This study examined copolymers synthesized from poly(trimethylene terephthalate) (PTT) and p-acetoxybenzoic acid using solution proton nuclear magnetic resonance (NMR) spectroscopy. Proton NMR spectra showed that these p-oxybenzoate (POB)/PTT copolyesters were almost random copolymers because the preference factor of POB bonded to another POB unit in these copolyesters is close to 1.0 with a POB content between 20 and 80 mol%.The melting and crystallization behaviors of these copolyesters were studied by differential scanning calorimetry (DSC). In the heating DSC scan of the POB rich composition, the endothermic peak is weaker because the enthalpy of fusion decreased due to a melting transition from a crystalline to anisotropy liquid state. Thermogravimetric analysis results indicated that the decomposition temperature (T_d) increased with POB content. The crystalline morphology of the copolyester was further investigated with a polarized optical microscope, indicating that the POB/PTT copolyesters with 60 mol% POB are highly anisotropic in the liquid state.     

Dual temperature‐ and pH‐sensitive hydrogels from interpenetrating networks and copolymerization of N‐isopropylacrylamide and sodium acrylate

Hydrogels responsive to both temperature and pH have been synthesized in the forms of sequential interpenetrating networks (IPNs) of N‐isopropylacrylamide (NIPAAm) and sodium acrylate (SA) and compared with the crosslinked random copolymers of N‐isopropylacrylamide and SA. Whereas the stimuli‐sensitive behaviors of copolymer hydrogels were strongly dependent on the ionic SA contents, the IPN hydrogels exhibited independent swelling and thermal behaviors of each network component. The sequences and media in the synthesis of IPNs influenced the swelling capacities of the IPNs, but not the temperature or pH ranges at which the swelling changes occurred. In IPNs, a more expanded primary gel network during the synthesis of the secondary network contributed to the better swelling of the final IPNs. Both the swelling and thermal behaviors of the IPNs suggest that poly(N‐isopropylacrylamide) and poly(sodium acrylate) are phase separated regardless of their synthesis conditions. The presence of the poly(sodium acrylate) network did not influence the temperature or the extent of phase transition of the poly(N‐isopropylacrylamide) network in the IPNs, but did improve the thermal stability of the IPNs. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3293–3301, 2004     

Study on hydration of poly(N-vinylcaprolactam) microgels by near-IR and mid-IR spectroscopy

Hydration of poly(N-vinylcaprolactam) microgels was investigated by near-infrared (NIR) and mid-infrared (MIR) spectroscopy. The thermosensitive microgels were prepared by emulsion polymerization, and turbidity, dynamic light scattering, and differential scanning calorimetry measurements were carried out. In MIR spectra, carbonyl bands consist of three components due to double, single, and zero hydrogen-bonding carbonyl groups as verified by density functional theory calculations. The relative intensities changed critically at the volume phase transition temperature upon heating. In NIR spectra, two absorbance peaks around 5,900?cm^?1 were observed, which can be assigned to the first overtone of C–H bands. Both of them undergo red shifts during the phase transition in a similar way to that of fundamental bands in MIR spectra. The result suggests that NIR spectroscopy may be a new general method that can provide new information for research on hydration of thermosensitive microgels.     

Curing kinetics and morphology of IPNs from a flexible dimethacrylate and a rigid epoxy via sequential photo and thermal polymerization

The curing kinetics and morphology of Interpenetrating polymer networks (IPNs) formed from a rigid epoxy resin thermally cured by an anhydride, and a photocured flexible dimethacrylate resin, have been studied by temperature ramping differential scanning calorimetry (DSC), near-infrared (NIR), and dynamic mechanical thermal analyzer (DMTA). This combination of cross-linkable resins permits the partial or complete cure of each component independent of the other. Also, since the monomers are polar but chemically dissimilar thermosetting resins, their IPNs should offer considerable variation in properties. DSC studies showed that the possible interactions between each component in the IPN could be minimized, but that the curing rate and conversion of the second polymerizing component was affected by vitrification, network topology, or phase separation in the IPN. NIR was also used to confirm that virtually independent cure was achievable by the combination of the thermal and photochemical methods. Dynamical mechanical thermal analysis was used to investigate the effect of curing one or both components and of order of curing on the phase morphology of the IPN. The modulus in the rubbery region also provided information on loop formation and co-continuity of each network component through the polymer matrix. The modulus and tan δ curves showed large differences in the glass transition region of the IPNs with different curing schedules, however phase separation occurred in all fully cured IPNs. These observations conflict with a previously advanced hypothesis that rapid polymerization and gelation of the last-cured component interlocks the two networks into a single phase structure and leads to the inclusion of a caveat that the components require sufficient attraction for interlocking of the networks to occur.