Synthesis and characterization of temperature-responsive copolymer of PELGA modified poly(N-isopropylacrylamide)

Novel amphiphilic PELGA modified temperature-responsive copolymer, [(poly(methoxyethylene glycol)-co-poly(lactic acid)-co-poly-(glycolic acid))acrylate-co-poly(N-isopropylacrylamide)-co-poly(N-hydroxymethylacrylamide)] (PELGAA-co-PNIPAAm-co-PNHMAAm) was synthesized by incorporating PELGA as the amphiphilic moiety into poly(N-isopropylamide) with various LA/GA ratios. Polymers obtained were characterized by FT-IR, GPC, ¹H-NMR and DSC. The lower critical solution temperature (LCST) of the copolymeric nanoparticles was 40±0.6 °C, the critical aggregation concentration (CAC) was 18 mg L⁻¹, and reversible change in nanoparticle size related to temperature was fluctuated between 210±10 and 109±26 nm, while change in zeta potential of the nanoparticles was between −36±6 and −26±4 mV. The transmission electron microscopy (TEM) images of nanoparticles were also presented.     

Preparation and characterization of hyperbranched polymer grafted mesoporous silica nanoparticles via self-condensing atom transfer radical vinyl polymerization

Hyperbranched poly(2-((bromobutyryl)oxy)ethyl acrylate) (HPBBEA) was grafted onto the exterior surface of mesoporous silica nanoparticles (MSNs) by surface-initiated self-condensing atom transfer radical vinyl polymerization (SCATRVP). The MSNs with ATRP initiator anchored on the exterior surface (MSN-Br) were prepared by the reaction of 5,6-dihydroxyhexyl-functionalized MSNs (MSN-OH) with α-bromoisobutyryl bromide. Afterwards, MSN-Br was utilized as initiator in the SCATRVP of inimer BBEA, resulting in core-shell nanoparticles with MSN core and HPBBEA shell (MSN-g-HPBBEA). The molecular weight of HPBBEA increased with the increasing ratio of BBEA to MSN-Br. In view of the high density of bromoester groups on the surface of HPBBEA shell, MSN-HPBBEA was used to initiate the successive polymerization of (2-dimethylamino-ethylmethacrylate) (DMAEMA), forming core-shell nanoparticles MSN-g-HPBBEA-g-PDMAEMA. The resultant products were characterized by FT-IR, NMR, HRTEM and thermogravimetric analysis (TGA), etc. The pH-responsive property of MSN-g-HPBBEA-g-PDMAEMA was characterized by measuring the hydrodynamics radius at different pH values, and this core-shell nanostructure may have potential applications in biomedicine and biotechnology.     

Real-time observation of coil-to-globule transition in thermosensitive poly(N-isopropylacrylamide) brushes by quartz crystal microbalance

Thermosensitive poly(N-isopropylacrylamide) (PNIPAm) brushes grafted on SiO₂-coated quartz crystal surface were prepared by the surface initiated radical polymerization. Using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), about 50 nm thickness of PNIPAm brushes were successfully formed. Quartz crystal microbalance with dissipation (QCM-D) is employed to investigate the collapse and swelling behavior of the PNIPAm brushes in water in real time. Both frequency and dissipation of PNIPAm layer were found to change gradually over the temperature range 15-50 °C, indicating that the brushes undergo a continuous transition. This continuous change is attributed to the nonuniformity and stretching of PNIPAm brushes as well as the cooperativity between collapse and dehydration transition.     

Synthesis and properties of polymer brush consisting of poly(phenylacetylene) main chain and poly(dimethylsiloxane) side chains

A novel polymer brush consisting of poly(phenylacetylene) (PPA) main chain and poly(dimethylsiloxane) (PDMS) side chains was synthesized by the polymerization of phenylacetylene-terminated PDMS macromonomer (M-PDMS). The macromonomer was prepared by the esterfication of monohydroxy-ended PDMS (PDMS-OH, degree of polymerization (DP) = 42) with p-ethynylbenzoic acid. The polymerization of M-PDMS using [(nbd)RhCl]₂/Et₃N catalyst led to polymer brush, poly(M-PDMS), with M_n up to 349?000 (DP of main chain 104). Poly(M-PDMS) with narrow molecular weight distribution (M_n = 39?900, M_w/M_n = 1.11) was obtained with a vinyl-Rh catalyst, [Rh{C(Ph)CPh₂}(nbd){P(4-FC₆H₄)₃}]/(4-FC₆H₄)₃P. Poly(M-PDMS)s were brown to orange viscous liquids and soluble in organic solvents such as toluene and CHCl₃. The UV-vis absorptions of poly(M-PDMS) were observed in the range of 350-525 nm, which are attributable to the PPA main chain.     

Effect of hydrophobic polystyrene microphases on temperature-responsive behavior of poly(N-isopropylacrylamide) hydrogels

Reversible addition-fragmentation chain transfer polymerization was employed to prepare the crosslinked poly(N-isopropylacrylamide)-graft-polystyrene networks (PNIPAAm-g-PS). Due to the immiscibility of PNIPAAm with PS, the crosslinked PNIPAAm-g-PS copolymers displayed the microphase-separated morphology. While the PNIPAAm-g-PS copolymer networks were subjected to the swelling experiments, it is found that the PS block-containing PNIPAAm hydrogels significantly exhibited faster response to the external temperature changes according to swelling, deswelling, and reswelling experiments than the conventional PNIPAAm hydrogels. The improved thermo-responsive properties of hydrogels have been interpreted on the basis of the formation of the specific microphase-separated morphology in the hydrogels, i.e., the PS blocks pendent from the crosslinked PNIPAAm networks were self-assembled into the highly hydrophobic nanodomains, which behave as the microporogens and thus promote the contact of PNIPAAm chains and water. The self-organized morphology in the hydrogels was further confirmed by photon correlation spectroscopy (PCS). The PCS shows that the linear model block copolymers of PNIPAAm-g-PS networks were self-organized into micelle structures, i.e., the PS domains constitute the hydrophobic nanodomains in PNIPAAm-g-PS networks.     

Microscopic implication of rapid shrinking of comb-type grafted poly(N-isopropylacrylamide) hydrogels

The shrinking mechanism of comb-type grafted poly(N-isopropylacrylamide) gel was investigated by small-angle X-ray scattering (SAXS). The SAXS reveals that the microdomain structure with characteristic spacing of 460 Å is developed in the comb-type grafted poly(N-isopropylacrylamide) gel during the shrinking process. These observations suggest that the freely mobile characteristics of the grafted chains are expected to show the rapid dehydration to make tightly packed globules with temperature, followed by the subsequent hydrophobic intermolecular aggregation of the dehydrated graft chains. The dehydrated grafted chains created the hydrophobic cores, which enhance the hydrophobic aggregation of the networks. These aggregations of the NIPA chains contribute to an increase in void volume, which allow the gel having a pathway of water molecules by the phase separation.     

Spectroscopic studies of polymer electrolytes based on poly(N-ethylethylenimine) and poly(N-methylethylenimine)

Poly(ethylethylenimine), PEEI, was prepared from poly(ethylenimine) by reductive alkylation with acetaldehyde. Samples of PEEI and poly(methylenimine), PMEI, complexed with LiCF₃SO₃ were prepared and characterized using differential scanning calorimetry and FT-IR. Small differences in the room temperature spectra of the two complexes were noted; these differences were due to the presence of a CH₂ group in the side chain of PEEI. The predominant form of cation-anion interactions was a contact ion pair. As the samples were heated, a transition from ion pairs to “free” ions was observed, with most of the change occurring between 140 and 150 °C in both PEEI and PMEI complexes. Thermal cycling established that the transition was irreversible in the time frame of the cycling experiments. Two-dimensional correlation spectroscopy did not show any significant intensity or frequency changes in bands sensitive to cation-polymer interactions during any heating or cooling cycle.     

Nanostructured polymer blend formed from poly(N-vinylpyrrolidone) and end-functional polystyrene

Phase structures of blends of poly(N-vinylpyrrolidone) (PVP) with SO₃H terminated polystyrene (PSS) were investigated. The PVP-PSS blends were macroscopically homogeneous, although the blends of PVP with polystyrene (PS) showed macroscopic phase separation. The PVP-PSS blends, however, showed two glass transitions indicating existence of two phases. Small-angle X-ray scattering measurements revealed the PVP-PSS blends formed mesomorphically ordered morphologies which change with variation of blend composition. The nano-organized phase separation in the PVP-PSS blends was caused due to hydrogen bonding of the PVP with the terminal SO₃H group of the PSS and repulsive interaction between PVP and main chain of the PSS.     

Novel thermo-responsive membranes composed of interpenetrated polymer networks of alginate-Ca and poly(N-isopropylacrylamide)

In this work, interpenetrated polymer networks (IPN) composed of alginate-Ca²⁺ and poly(N-isopropylacrylamide), PNIPAAm, were synthesized and their water uptake capability was measured at temperatures from 25 to 40 °C and compared to that of pure alginate-Ca²⁺ hydrogels without PNIPAAm. A sharp decrease of WU was observed when IPN hydrogels are heated above 32-33 °C. The phenomenon is associated to a drastic shrinking of hydrogels. At temperatures above 32 °C the PNIPAAm chains collapse, contracting their network and pulling back the alginate-Ca²⁺ network. The rate of shrinking depends of the heating rate. The phenomenon is more effective and faster in IPN containing lower amount of alginate-Ca²⁺. The shrunken IPN hydrogels can be re-swollen but the expansion is slower than the shrinking. The diffusion of Orange II dye through the membrane of IPN hydrogels decreases if the temperature is raised up to 35 °C. The shrinking results in a decrease of the average pores size that makes more difficult the diffusion of Orange II. The average pore size was evaluated in several stages by analysis of SEM micrographs of freeze dried samples: 102.0±14.3 μm at 25 °C, 15.7±5.4 μm at 33 °C and 0.4±0.3 μm at 40 °C. Below the LCST of PNIPAAm, the IPN hydrogels exhibit a morphology characterized by open pores but above the LCST their surface becomes more regular and compact. As a consequence, an increase of the apparent activation energy for permeability, , of Orange II is measured.     

Synthesis and application as polymer electrolyte of hyperbranched copolyethers derived from cationic ring-opening polymerization of 3-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}methyl- and 3-hydroxymethyl-3′-methyloxetane

A kind of novel hyperbranched copolyethers intending for the solid polymer electrolyte was synthesized via the cationic ring-opening polymerization of 3-{2-[2-(2-methoxyethoxy)ethoxy]-ethoxy}methyl-3′-methyloxetane (MEMO) and 3-hydroxymethyl-3′-methyloxetane (HMO) in the presence of BF₃·Et₂O as an initiator. Herein HMO was employed to create the hyperbranched structure, whereas MEMO was responsible for the ionic transportation of the resulting copolymers. The terminal structure featured by a cyclic fragment was definitely detected by MALDI-TOF measurement. The degree of branching of the copolymers was calculated by means of ¹³C NMR spectra. The DSC analysis implied that they hold the excellent segment motion performance and perfectly amorphous state beneficial for the ionic transportation. The ionic conductivity measurements showed that the sample HMO 30 reaches a maximum ionic conductivity of 8.0 × 10⁻⁵ S/cm at 30 °C and 7.4 × 10⁻⁴ S/cm at 80 °C, respectively, after doping with lithium salt LiTFSI. Moreover, the TGA assay exhibited that these hyperbranched copolymers possess the higher thermostability as compared with their liquid counterparts.     

Postgrafting of vinyl polymers onto hyperbranched poly(amidoamine)-grafted nano-sized silica surface

To prepare polymer-grafted nano-sized silica with hydrophilic core and hydrophobic shell and with higher percentage of grafting, the postgraft polymerization of vinyl polymers onto hyperbranched poly(amidoamine)-grafted (PAMAM-grafted) nano-sized silica initiated by the system consisting of Mo(CO)₆ and terminal trichloroacetyl groups of PAMAM-grafted silica was investigated. The introduction of trichloroacetyl groups onto PAMAM-grafted silica surfaces was readily achieved by the reaction of trichloroacetyl isocyanate with terminal amino groups of PAMAM-grafted silica. It was found that the polymerization of vinyl monomers, such as methyl methacrylate (MMA), styrene, and glycidyl methacrylate (GMA) was successfully initiated by the system consisting of Mo(CO)₆ and terminal trichloroacetyl groups of PAMAM-grafted silica. In the polymerization, the corresponding vinyl polymers were effectively postgrafted onto PAMAM-grafted silica, based on the propagation of polymer from surface radicals formed by the reaction of terminal trichloroacetyl groups with Mo(CO)₆: the percentage of PMMA postgrafting onto PAMAM-grafted silica reached to 400% after 30 min, but the formation of gel was observed after 35 min. The formation of gel tends to decrease by use of hyperbranched PAMAM-grafted silica with higher percentage of grafting. The vinyl polymer-postgrafted nano-sized silica gave a stable colloidal dispersion in various organic solvents.     

Synthesis and properties of semi-crystalline hyperbranched poly(ester-amide) grafted with long alkyl chains used for UV-curable powder coatings

A series of semi-crystalline hyperbranched poly(ester-amide)s by modifying hydroxyl end groups of hyperbranched poly(ester-amide) (HP) with IPDI-C18 and IPDI-HEA in different ratios were synthesized and characterized by FTIR, NMR and GPC. Their crystallization behaviors and thermal properties determined by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) showed that the high substitution degree of hydroxyl groups of hyperbranched poly(ester-amide) (HP) with IPDI-C18 resulted in higher degree of crystallization and thus glass transition temperature (T_g) up to 43 °C. The photopolymerization kinetics investigated by photo-DSC showed that the obtained semi-crystalline hyperbranched resins have high photopolymerization rate and final unsaturation conversion, which is very promising for UV-curable powder coating applications.     

Synthesis, optical and electrochemical properties of new hyperbranched poly(triphenylamine amide)s

A new triphenylamine-containing AB₂ type monomer with one carboxylic acid and two amino groups, 4-(bis(4-aminophenyl)amino)benzoic acid (3), was synthesized and used for the preparation of hyperbranched poly(triphenylamine amide)s. The self-polycondensation of the AB₂ monomer (3) afforded hyperbranched poly(triphenylamine amide) with amino end groups. The molecular weight of the hyperbranched poly(triphenylamine amide) was 21,000 Da determined by light scattering. End-capped hyperbranched polyamides were isolated by the chemical modification of unreacted amino groups with various acid chlorides. All the hyperbranched poly(triphenylamine amide)s exhibit excellent solubility in organic solvents such as NMP, DMF, DMSO, and DMAc at room temperature. The viscosities of hyperbranched poly(triphenylamine amide)s are as low as about 0.15 dL/g due to their dendritic structures. Poly(triphenylamine amide)s end-capped with rigid benzene ring have higher thermal stability than those with amino or aliphatic end groups. The photoluminescence of the hyperbranched polyamides is blue-yellow emissions around 430-510 nm. The energy gaps of the hyperbranched poly(triphenylamine amide)s with different end groups are about 2.93 eV and are independent on the end groups, but the HOMO and LUMO energy levels are dependent on the end groups.     

Thermoresponsive core-shell-corona micelles of poly(ethyleneglycol)-b-poly(N-isopropylacrylamide)-b-polystyrene

Core-shell-corona micelles with a thermoresponsive shell self-assembled by triblock copolymer of poly(ethyleneglycol)-b-poly(N-isopropylacrylamide)-b-polystyrene (PEG₄₅-b-PNIPAM₁₆₈-b-PS₄₆) are studied by ¹H NMR, light scattering and atomic force microscopy. The thermoresponsive triblock copolymer, which has a relatively short hydrophobic PS block, can disperse in water at room temperature to form core-shell-corona micelles with the hydrophobic PS block as core, the thermoresponsive PNIPAM block as shell and the hydrophilic PEG block as corona. At temperature above lower critical solution temperature (LCST) of the PNIPAM block, the PNIPAM chains gradually collapse on the PS core to shrink the size and change the structure of the resultant core-shell-corona micelles with temperature increasing. It is found that there possibly exists an interface between the PNIPAM shell and PEG corona of the core-shell-corona micelles at temperature above LCST of the PNIPAM block.     

Dependence of shrinking kinetics of poly(N-isopropylacrylamide) gels on preparation temperature

Preparation temperature dependence of equilibrium swelling degree and shrinking kinetics of poly(N-isopropylacrylamide) gel has been investigated by optical microscopic measurements. The degree of swelling, d/d₀, at 20 °C was found to be strongly dependent on the preparation temperature, T_prep, where d and d₀ are the diameter of gel during observation and preparation, respectively. The value of d/d₀ was about 1.2 for T_prep=20 °C, but steeply increased by approaching the phase separation temperature ≈32.0 °C. Above 32.0 °C, d/d₀ decreases stepwise to 1.46. This upturn in d/d₀ was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing T_prep. Though the shrinking half-time, t_1/2, of gel was on the order of 500 min for T_prep≤20 °C, t_1/2 decreased to 2 min for T_prep≥26 °C. Hence, a rapid shrinking was attained by simply increasing T_prep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium.     

In situ grafting of carboxylic acid-terminated hyperbranched poly(ether-ketone) to the surface of carbon nanotubes

Carboxylic acid-terminated hyperbranched poly(ether-ketone)s (HPEKs) were successfully grafted onto the surfaces of single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) to afford HPEK-g-SWNT and HPEK-g-MWNT nanocomposites. They were prepared via in situ polymerization of 5-phenoxyisophthalic acid as an AB₂ monomer for the HPEK in the presence of SWNT or MWNT in polyphosphoric acid (PPA)/phosphorous pentoxide (P₂O₅) medium. The resultant nanocomposites were homogeneously dispersed in various common polar aprotic solvents as well as in concentrated ammonium hydroxide. The experimental results from Soxhlet extraction, solubility enhancement, elemental analysis (EA), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) provided clear evidences for grafting of hyperbranched polymers onto the surfaces of corresponding CNT's. Achieving enhanced solubility of CNT's in common organic solvents via the functionalization of CNT's is a key step for CNT's to be used in various application-specific purposes. The results could potentially envision to the area of CNT researches via the efficient introduction of three-dimensional globular dendritic macromolecules as increasing solubility, available multi-functionality, reactivity, processability, and also biocompatibility.     

Effect of salt on the elastic modulus of poly(N-isopropylacrylamide) gels

The measurement of tensile modulus of poly(N-isopropylacrylamide) (PNIPA) gel in the solution of NaCl, NaI, LiNO₃ and NaNO₃ was carried out. It was confirmed that the tensile modulus of PNIPA gel in the solution of salt depends on the volume of gel regardless of the kind and concentration of salts. This result leads us to the conclusion that the addition of salt effect only on the mixing contribution to the Flory's type free energy of gel especially in the swollen state. Therefore, our result is in agreement with a recent remarkable discovery that the volume of PNIPA gel depends only on the chemical potential of water in spite of the kind of additives. On the other hand, it was found that on the volume phase transition point and in the deswollen state, the elasticity of PNIPA gel depended on the concentration and kind of the salt because the viscoelasticity emerged due to the shrinkage of polymer network.     

Coating of gold nanoparticles by thermosensitive poly(N-isopropylacrylamide) end-capped by biotin

Gold nanoparticles (NPs) were prepared by reduction of HAuCl₄ in aqueous solution and stabilized by poly(N-isopropylacrylamide) (PNIPAM). PNIPAM was prepared by two distinct routes: (i) conventional free-radical polymerization leading to polymer without any reactive end-group, and (ii) Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization with 2-dodecylsulfanylthiocarbonylsulfanyl-2-methyl propionic acid (DMP) as a RAFT agent. PNIPAM with low polydispersity was then end-capped by an α-carboxylic acid and an ω-trithiocarbonate that was converted into an ω-thiol upon hydrolysis. This hetero-telechelic polymer was analyzed by mass spectroscopy, size exclusion chromatography (SEC) and ¹H NMR. Even without thiol end-group, known for chemisorption onto gold, PNIPAM was effective in stabilizing gold NPs (∼1-5 nm). The thermosensitivity of PNIPAM at the surface of gold NPs was, however, dependent on the molecular weight of the chains. Finally, the α-carboxyl end-group of PNIPAM was used to anchor biotin, which is indeed known for complexation with avidin, which is a possible strategy for the coated gold NPs to be involved as building blocks in supramolecular assemblies. TEM and UV-vis spectroscopy were used to characterize the gold nanoparticles.     

Biodegradable and thermostable synthetic hyperbranched poly(urethane-urea)s as advanced surface coating materials

Aliphatic hyperbranched poly(urethane-urea)s with different weight percentages of branch generating moiety were synthesized by a one pot A₂ + BC₂ approach. Isophorone diisocyanate was used as the A₂ type monomer, while a tri-functional dihydroxyamine compound synthesized from ?-caprolactam and diethanol amine acted as the BC₂ monomer. Evidence supporting the hyperbranched structure of the synthesized poly(urethane-urea) was obtained from ¹H NMR spectra. FTIR study confirmed the nature and extent of hydrogen bonding present in this novel macromolecule. A Gaussian band fitting procedure of the IR band at amide-I region showed that the extent of hydrogen bonding increases with the increase of weight percentage of the tri-functional compound. The tensile strength, elongation at break, impact resistance, scratch hardness and gloss followed an increasing trend with the same. The thermal degradation of the hyperbranched poly(urethane-urea) was found to be dependent on the weight percentage of the BC₂ type moiety. The kinetics of thermal degradation studied by the Ozawa method showed that the activation energy required for thermal degradation of hyperbranched polymer is higher than its linear polyurethane analog. The synthesized polymer was found to be biodegradable by Pseudomonas aeruginosa bacteria. The study showed superiority of the hyperbranched structure over the linear one. Thus the results indicated the potential usage of the studied hyperbranched poly(urethane-urea) as an advanced surface coating material.