Spectral time moment analysis of microgel deswelling. Effect of the heating rate

Microgel nanoparticles were synthesized in aqueous solutions of neutral polymer hydroxypropylcellulose (HPC) through the self-association of amphiphilic HPC molecules and the subsequent cross linking at room temperature. Dynamic Light Scattering was used to study the transport properties of HPC microgels below and above the volume phase transition. Highly nonexponential, multimodal microgel spectra were observed and successfully analyzed by spectral time moment analysis. This article expands earlier results and focuses on the effect of the heating rate on microgel deswelling. During the fast heating two identified microgel modes with apparent hydrodynamic radii (R_H) of 25–30 nm and 400–650 nm collapse into one mode with R_H = 100–150 nm. This indicates the shrinkage of microgel size distribution and an apparent decrease in the radius of larger microgels. During the slow heating, however, both microgel-identified modes remain present above T_c. Although equally represented below the transition, the dominance of larger microgels' mode increases almost two fold with rising temperature above 40°C. Moreover, R_H for this mode increases from 250–300 nm to about 800–850 nm with a multi-step temperature change from 40 to 42.5°C, indicating the growth (and not shrinkage) of microgels. The second mode is represented by the temperature independent R_H, but its contribution goes down from about 50% to less than 10%. © 2008Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2792–2802, 2008     

N‐vinylcaprolactam‐based microgels: Synthesis and characterization

Poly(N‐vinylcaprolactam) (PVCL) is well known for its thermoresponsive behavior in aqueous solutions. PVCL combines useful and important properties; it is biocompatible polymer with the phase transition in the region of physiological temperature (32–38 °C). This combination of properties allows consideration of PVCL as a material for design biomedical devices and use in drug delivery systems. In this work, PVCL based temperature‐sensitive crosslinked particles (microgels) were synthesized in a batch reactor to analyze the effect of the crosslinker, initiator, surfactant, temperature, and VCL concentration on polymerization process and final microgels characteristics. The mean particle diameters at different temperatures and the volume phase‐transition temperature of the final product were analyzed. To obtain information about the inner structure of microgel particles, semicontinuous polymerizations were carried out and the evolution of the hydrodynamic average particle diameters at different temperatures of the microgel synthesized was investigated. In the case of microgel particles obtained in a batch reactor the size and the swelling‐de‐swelling behavior as a function of the temperature of the medium can be tuned by modulating the reaction variables. When the microgel particles were synthesized in a semibatch reactor different swelling‐de‐swelling behaviors were observed depending on particles inner structure. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2510–2524, 2008     

Volume transition and internal structures of small poly(N‐isopropylacrylamide) microgels

Monodispersed poly(N‐isopropylacrylamide) (PNIPAM) nanoparticles, with hydrodynamic radius less than 50 nm at room temperature, have been synthesized in the presence of a large amount of emulsifiers. These microgel particles undergo a swollen–collapsed volume transition in an aqueous solution when the temperature is raised to around 34 °C. The volume transition and structure changes of the microgel particles as a function of temperature are probed using laser light scattering and small angle neutron scattering (SANS) with the objective of determining the small particle internal structure and particle–particle interactions. Apart from random fluctuations in the crosslinker density below the transition temperature, we find that, within the resolution of the experiments, these particles have a uniform radial crosslinker density on either side of the transition temperature. This result is in contrast to previous reports on the heterogeneous structures of larger PNIPAM microgel particles, but in good agreement with recent reports based on computer simulations of smaller microgels. The particle interactions change across the transition temperature. At temperatures below the transition, the interactions are described by a repulsive interaction far larger than that expected for a hard sphere contact potential. Above the volume transition temperature, the potential is best described by a small, attractive interaction. Comparison of the osmotic second virial coefficient from static laser light scattering at low concentrations with similar values determined from SANS at 250‐time greater concentration suggests a strong concentration dependence of the interaction potential. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 849–860, 2005     

Conformational change of proline residues observed in swollen and shrunken phases: γ‐Ray synthesis and variable temperature circular dichroism spectra of a thermo‐responding polymer,poly(acryloyl‐Pro‐OMe)

Poly(acryloyl‐L ‐proline‐methyl ester) ( 1 ) has optically active side chain, and constitutes thermoresponsive hydrogels upon crosslinking. In this study, we have prepared uncrosslinked polymer of 1 with a 10‐kGy irradiation dose of γ‐ray. For this polymer, 1 , variable temperature circular dichroism (CD) and ¹H NMR spectra have been studied in the range of 0–30 °C. The intense CD spectrum at 0 °C suggests that the side chains in 1 have an ordered orientation. The CD intensity decreases gradually with increasing temperature. The decreased intensity of CD spectra indicates that the disordering occurs for the side‐chain orientation. The CD band shape changes discontinuously at 20 °C. In the ¹H NMR spectra, signals disappear above 20 °C. These spectral change at 20 °C indicate that the phase transition occurs at around 20 °C from swollen to shrunken phase. Even after the phase transition, the CD spectra are still changing with isochromic point at 212 nm. It appears that the side‐chain conformation is still changing from one state to the other state in the shrunken phase polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4524–4530, 2000     

Preparation and characterization of pH‐responsive and thermoresponsive hybrid microgel particles with gold nanorods

We report on pH‐responsive and thermoresponsive hybrid materials based on the assembly of gold nanorods, Au NRs, into multiresponsive, crosslinked copolymer microgel particles. These microgel particles were prepared by the surfactant‐free emulsion polymerization of N‐isopropylacrylamide and acrylic acid using N, N′‐methylene bis‐acrylamide as a crosslinker, which produces particles measuring approximately 160 nm that are interconnected to one other. Cetyltrimethyl ammonium bromide‐stabilized Au NRs were also prepared independently using a seed‐mediated growth method and then loaded into swollen, deprotonated, acrylic acid‐containing microgel particles using the electrostatic interactions between the oppositely charged particles. Transmission electron micrographs of the as‐prepared hybrid Au NR–microgel particles confirmed that the Au NRs were attached to the surface of the microgel particles. The size‐dependent temperature‐responsive characteristics of the hybrid microgel particles were studied by dynamic light scattering, and it was found that as the temperature increased across the phase transition temperature, the particle size decreased to 56% of the original volume. The thermoresponsive and pH‐responsive optical properties of the hybrid microgel particles were also systematically investigated. The thermo‐ and pH‐induced shrinkage of the microgel led to an increase in the UV–vis absorption intensity and caused a significant blue shift in the longitudinal surface plasmon bands of the Au NRs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Structure and dynamics of solutions of a polystyrene–polybutadiene pentablock copolymer in a styrene‐selective solvent

We have examined solutions of a polystyrene–polybutadiene pentablock copolymer in 1,4‐dioxane, a slightly selective solvent for polystyrene and a θ solvent for polybutadiene, with static light scattering (SLS), dynamic light scattering (DLS), and small‐angle neutron scattering (SANS). The SANS data have been analyzed with the Percus–Yevick model to represent the scattering from interacting cores, approximated as hard spheres, and with a Lorentzian function to represent the scattering from unassociated and associated polymer chains. The SANS data at 25 °C clearly reveal interacting domains, approximately 6 nm in radius, formed by the association of the insoluble polybutadiene block in the 20% sample. The 4% sample does not show such domains, whereas the 7% sample represents an intermediate situation, with both unassociated polymer and associated polymer. At higher temperatures, the domains dissolve. The DLS data for samples with concentrations of 2–22% show two diffusive modes: a fast mode corresponding to the cooperative dynamics of concentration fluctuations and a slow mode corresponding to the diffusion of clusters. The large length‐scale heterogeneities, indicated by the strong angular dependence of SLS, implies that the small microdomains of about 10–15 polybutadiene blocks are bridged by the polystyrene chains, forming large aggregates with randomly distributed crosslinks on length scales much larger than the domain size. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2807–2816, 2002     

Preparation of Acrylic Microgels by Modified Microemulsion Polymerization and Phase Inversion

Semi‐transparent reactive microgel in nanosize has been prepared by modified microemulsion polymerization using a common emulsifier, crosslinking agent and functional monomer. The microgels are translucent reactive nanoparticles, with the size of 40–100 nm, consisting of inner‐crosslinked polymer up to 40%. FT‐IR proved the functional groups, such as epoxy and hydroxy, are on the surface of the microgel nanoparticles. Rheological detection demonstrated the apparent pseudoplasticity of the non‐aqueous microgel dispersion prepared by the phase transfering from the O/W microlatexes.     

Crosslinkable polyurethane bearing a methacrylate structure in the side chain

A polyurethane bearing methacrylate groups through urethane linkages was prepared by the addition of 2‐methacryloyloxyethyl isocyanate to the hydroxyl groups in poly(hydroxyurethane) prepared by the polyaddition of a bifunctional cyclic carbonate with 1,12‐diaminododecane. The urethanization proceeded quantitatively in the presence of a catalytic amount of di‐n‐butyltin dilaurate at an ambient temperature, whereas a crosslinked product was obtained from the reaction at 60 °C. The resulting linear polyurethane, bearing a methacrylate structure, was thermally crosslinkable. Its radical copolymerization with vinyl‐type monomers afforded the corresponding crosslinked polymers, whose low glass transition temperatures suggested the flexibility of the polymer chains in the crosslinked product. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3400–3407, 2007     

Glass transition broadening via nanofiller-contiguous polymer network in aromatic thermosetting copolyester nanocomposites

The glass transition is a genuine imprint of temperature-dependent structural relaxation dynamics of backbone chains in amorphous polymers, which can also reflect features of chemical transformations induced in macromolecular architectures. Optimization of thermophysical properties of polymer nanocomposites beyond the state of the art is contingent on strong interfacial bonding between nanofiller particles and host polymer matrix chains that accordingly modifies glass transition characteristics. Contemporary polymer nanocomposite configurations have demonstrated only marginal glass transition temperature shifts utilizing conventional polymer matrix and functionalized nanofiller combinations. We present nanofiller-contiguous polymer network with aromatic thermosetting copolyester nanocomposites in which carbon nanofillers covalently conjugate with cure advancing crosslinked backbone chains through functional end-groups of constituent precursor oligomers upon an in situ polymerization reaction. Via thoroughly transformed backbone chain configuration, the polymer nanocomposites demonstrate unprecedented glass transition peak broadening by about 100 °C along with significant temperature upshift of around 80 °C. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1595–1603     

A simple FTIR spectroscopic method for the determination of the lower critical solution temperature of N‐isopropylacrylamide copolymers and related hydrogels

Linear and crosslinked polymers based on N‐isopropylacrylamide (NIPAAm) exhibit unusual thermal properties. Aqueous solutions of poly(N‐isopropylacrylamide) (PNIPAAm) phase‐separate upon heating above a lower critical solution temperature (LCST), whereas related hydrogels undergo a swelling–shrinking transition at an LCST. A linear copolymer made of NIPAAm/acryloxysuccinimide (98/2 mol/mol) and two hydrogels with different hydrophilicities were prepared. Fourier transform infrared (FTIR) spectroscopy was employed to determine the transition temperature and provide insights into the molecular details of the transition via probing of characteristic bands as a function of temperature. The FTIR spectroscopy method described here allowed the determination of the transition temperature for both the linear and crosslinked polymers. The transition temperatures for PNIPAAm and the gel resulting from the crosslinking with polylysine or N,N′‐methylenebisacrylamide (MBA) were in the same range, 30–35 °C. For the gels, the transition temperature increased with the hydrophilicity of the polymer matrix. The spectral changes observed at the LCST were similar for the free chains and the hydrogels, implying a similar molecular reorganization during the transition. The C H stretching region suggests that the N‐isopropyl groups and the backbone both underwent conformational changes and became more ordered upon heating above the LCST. An analysis of the amide I band suggests that the amide groups of the linear polymer were mainly involved in hydrogen bonding with water molecules below the LCST, the chain being flexible and disordered in a water solution. During the transition, around 20% of these intermolecular hydrogen bonds between the polymer and water were broken and replaced by intramolecular hydrogen bonds. Similar changes were also observed at the LCST of a gel crosslinked with MBA. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 907–915, 2000     

High performance thermosets from a curable Diels–Alder polymer possessing benzoxazine groups in the main chain

A high‐molecular‐weight polymer (PBz) possessing reactive benzoxazine groups in the main chain was prepared through the Diels–Alder reaction using bis(3‐furfuryl‐3,4‐dihydro‐2H‐1,3‐benzoxazinyl)isopropane (BPA‐FBz) and bismaleimide (BMI) as monomers. The chemical structure of PBz is characterized with FTIR and ¹H NMR. The polymer PBz was further thermally reacted with a high performance polymer (PBz‐R) through the ring‐opening addition reaction of benzoxazine groups and the addition reaction of maleimide groups. PBz‐R exhibit a high glass transition temperature of 242 °C, good thermal stability, high flame retardancy, high mechanical strength, and great flexibility. Another crosslinked polymer (PBz‐BR) curing from the mixture of BPA‐FBz and BMI was also prepared. The properties of PBz‐BR are also attractive but, however, not as good as what observed with PBz‐R. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6509–6517, 2008     

Shape memory biomaterials prepared from polyurethane/ureas containing sulfated glucose

Covalently crosslinked polyurethane/urea polymers were synthesized using diamine monomers modified with pendant glucose groups and 2,4‐toluene diisocyanate, poly(ethylene glycol) (PEG), and 1,1,1‐tris(hydroxymethyl)ethane (triol) comonomers. The polymers showed shape memory behavior with a switching temperature dependent on the glass transition temperature. The glass transition temperature is tuned by varying the mole ratio between the glucose‐diamine and PEG used in the polymerization. Increasing PEG content resulted in decreasing glass transition temperature, and a glass transition temperature of 39 °C, close to physiological temperatures, was obtained. The fixed shape showed gradual shape recovery behavior, but a fixity of 70% was achieved when the material was stored at 25 °C. The polymer recovered to the permanent shape when heated to 50 °C. Finally, the surface of a film of the polymer can be sulfated to achieve increased blood‐compatibility without sacrificing the shape memory properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2252–2257     

Ultrafine hydrogel fibers with dual temperature‐ and pH‐responsive swelling behaviors

Ultrafine hydrogel fibers that were responsive to both temperature and pH signals were prepared through the electrospinning of poly(N‐isopropylacrylamide) (PNIPAAm) and poly(acrylic acid) mixtures in dimethylformamide. Both the diameters (700 nm to 1.2 μm) and packing of the fibers could be controlled through changes in the polymer compositions and PNIPAAm molecular weights. These fibers were rendered water‐insoluble by the addition of either Na₂HPO₄ or poly(vinyl alcohol) (PVA) to the solution, followed by the heat curing of the fibers. The fibers crosslinked with Na₂HPO₄ swelled to 30–120 times in water; this was significantly higher than the swelling of those crosslinked with PVA. The PVA‐crosslinked hydrogel fibers, however, exhibited faster swelling kinetics; that is, they reached equilibrium swelling in less than 5 min at 25 °C. They were also more stable after 1 week of water exposure; that is, they lost less mass and retained their fibrous form better. All the hydrogel fibers showed a drastic increase in the swelling between pH 4 and 5. The PVA‐crosslinked hydrogel fibers exhibited distinct temperature‐responsive phase‐transition behavior of PNIPAAm, whereas the Na₂HPO₄‐crosslinked hydrogel fibers showed altered two‐stage phase transitions that reflected side‐chain modification of PNIPAAm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6331–6339, 2004     

Neutral polymer slow mode and its rheological correlate

Quasi‐elastic light scattering spectroscopy intensity–intensity autocorrelation functions [S(k,t)] and static light scattering intensities of 1 MDa hydroxypropylcellulose in aqueous solutions were measured. With increasing polymer concentration, over a narrow concentration range, S(k,t) gained a slow relaxation. The transition concentration for the appearance of the slow mode (c^t) was also the transition concentration for the solution‐like/melt‐like rheological transition (c⁺) at which the solution shear viscosity [η_p(c)] passed over from a stretched exponential to a power‐law concentration dependence. To a good approximation, we found c^t[η] ≈ c⁺[η] ≈ 4, [η] being the intrinsic viscosity. The appearance of the slow mode did not change the light scattering intensity (I): from a concentration lower than c^t to a concentration greater than c^t, I/c fell uniformly with increasing concentration. The slow mode thus did not arise from the formation of compact aggregates of polymer chains. If the polymer slow mode arose from long‐lived structures that were not concentration fluctuations, the structures involved much of the dissolved polymer. At 25 °C, the mean relaxation rate of the slow mode approximately matched the relaxation rate for the diffusion of 0.2‐μm‐diameter optical probes observed with the same scattering vector. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 323–333, 2005     

Synthesis and properties of microgel bearing a mercapto group

Vinylbenzyl S-thioacetate ( 1 ) was prepared from thioacetic acid and chloromethylstyrene. Although bulk polymerization of 1 afforded a crosslinked polymer, solution polymerization in chlorobenzene afforded a corresponding soluble polymer. The S-thioacetate group did not react during the radical polymerization of 1 . Bulk copolymerization of 1 with styrene afforded a soluble copolymer when the feed ratio of 1 was lower than 30 mol %. Soap-free emulsion copolymerization of 1 , St, divinylbenzene, and 2-hydroxyethyl methacrylate (66 : 28 : 1 : 5) was carried out in water using 2,2′-azobis (N,N′-dimethyleneisobutyramidine) dichloride as an initiator to afford uniform spherical microgel 2 , whose average diameter was 135 nm. Aminolysis of 2 with an excess amount of butylamine in the presence of sodium tetrahydridoborate followed by treatment with hydrochloric acid resulted in complete removal of the acetyl group to give a slightly distorted spherical microgel (MG-SH) bearing mercapto group. The average diameter of MG-SH was 165 nm. Trans-esterification of p-nitrophenyl acetate ( 3 ) in the presence of triethylamine was efficiently accelerated by the addition of MG-SH. The radical polymerization of methyl methacrylate (MMA) in the presence of suspended MG-SH in chlorobenzene afforded the MMA-grafted microgel. Although MG-SH is a crosslinked gel, it acts as a soluble polymer bearing mercapto group. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1443–1451, 1997     

Relaxational mode structure for optical probe diffusion in high molecular weight hydroxypropylcellulose

We studied translational diffusion of dilute monodisperse spheres (diameters 14 < d < 455 nm) in aqueous 1 MDa hydroxypropylcellulose (0 ≤ c ≤ 7 g/L) at 25°C using quasielastic light scattering. Spectra are highly bimodal. The two spectral modes (“slow,” “fast”) have different physical properties. Probe behavior differs between small (d < R_h) and large (d ≥ R_g) probes; R_h and R_g are the matrix polymer hydrodynamic radius and the radius of gyration, respectively. We examined the dependences of spectral lineshape parameters on d, c, scattering vector q, and viscosity η for all four probe-size and mode-type combinations. We find three time scale-separated modes: (1) a large-probe slow mode has properties characteristic of particle motion in a viscous medium; (2) a large-probe fast mode and small-probe slow modes share the same time scale, and have properties characteristic of probe motion coupled to internal chain dynamics; and (3) a small-probe fast mode has properties that can be attributed to the probe sampling local chain relaxations. In the analysis, we also attempted to apply the coupling/scaling (CS) model of Ngai and Phillies [Ngai, K. L., Phillies, G. D. J. J. Chem. Phys., 105 , 8385 (1996)] to analyze our data. We find that the second mode is described by the coupling/scaling model for probe diffusion; the first and third modes do not follow the predictions of this model. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3087–3100, 1998     

Synthesis of chromophore‐embedded polyurethanes for electrooptical materials

We carried out the polyaddition of dye‐embedded diols with diisocyanates to obtain novel nonlinear optical (NLO) polyurethanes, where the NLO units were embedded in the polymer backbone. The obtained polymers showed high glass‐transition temperatures (138–184 °C) and thermal stability (temperature of 10% weight loss under nitrogen = 227–287 °C). The λ maximum of the polymers was 521–556 nm. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2620–2624, 2001     

Amphiphilic PEG/alkyl‐grafted comb polylactides

Amphiphilic polylactides (PLAs) with well‐defined architectures were synthesized by ring‐opening polymerization of AB monomers (glycolides) substituted with both a long chain alkyl group and a triethylene glycol segment terminated in either a methyl or benzyl group. The resulting amphiphilic PLAs had number average molecular weights >100,000 g/mol. DSC analysis revealed a first‐order phase transition at ～ 20 °C, reflecting the crystalline nature of the linear alkyl side chains. Polymeric micelles were prepared by the solvent displacement method in water. Dynamic light scattering measurements support formation of a mixture of 20‐nm‐diameter unimolecular micelles and 60‐nm particles comprised of an estimated 25 polymer molecules. UV–vis characterization of micelles formed from acetone–water solutions containing azobenzene confirmed encapsulation of the hydrophobic dye, suggesting their potential as new amphiphilic PLAs as drug delivery vehicles. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5227–5236, 2007     

Side‐chain conformational changes during the thermoshrinking process: γ‐Ray polymerization and spectroscopic study of uncrosslinked poly(methacryloyl‐Ala‐OMe)

Poly(methacryloyl‐L ‐alanine‐methyl ester) (1) has an optically active side chain and consists of thermoshrinking hydrogels upon crosslinking. We synthesized an uncrosslinked polymer of 1 by the γ‐ray polymerization method. For the prepared polymer, variable‐temperature circular dichroism (CD) and ¹H NMR spectra were studied, and we found conformational changes in the optically active side chains during the thermally induced phase transition. Intense CD spectra reveal ordered conformation in the side chain of 1 below the phase transition temperature (∼28 °C). A well‐resolved ¹H NMR spectrum of 1 at 0 °C shows that the conformational angles in the polymer side chain are fixed at low‐energy minima. With increasing temperature, the frozen side chain starts rotating vigorously and takes an unordered orientation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2671–2677, 2000     

Synthesis and characterization of poly(vinyl alcohol)‐graft‐[poly(D,L‐lactide)/poly(D,L‐lactide‐co‐glycolide)] hydrogels

Poly(D ,L ‐lactide) and poly(D ,L ‐lactide‐co‐glycolide) with various composition and with one methacrylate and one carboxylate end group were synthesized and grafted onto poly(vinyl alcohol) (PVA) via the carboxylate group. The graft copolymers were crosslinked via the methacrylate groups using a free radical initiator. The polymer networks were characterized by means of NMR and studied qualitatively by means of IR spectroscopy. The influence of the glycolide content in the polyester grafts and of the number of ester units in the grafts on thermal properties and swellability were studied as well. The high swellability in water is characteristic of all hydrogels. Differential scanning calorimetry (DSC) showed a single glass transition temperature that occurs in the range between 51 and 69 °C. Thermogravimetric analysis (TGA) of the networks showed the main loss in weight in the temperature range between 290 and 370 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4536–4544, 2007