The imaging of nano and micro globules of short linear thermo responsive polyacrylamides formed above the lower critical solution temperature

An imaging method has been developed to examine thermo responsive polymer coagulates by optical and electron microscopy. Poly-N-isopropylacrylamide (PNIPAM), poly-N-dimethylacrylamide (PDMAM) and a 1:1 PNIPAM-PDMAM copolymer were encapsulated in a gelatin matrix as coagulates above the lower critical solution temperature (LCST), and subsequently examined by optical and electron microscopy. The linear macromolecules PNIPAM and PDMAM were synthesized by chain transfer polymerization with mercaptopropionic acid (3-MPA) as chain transfer reagent. The resulting polymers have an average molar mass of ∼1800 g/mol and low polydispersity. The LCST of thermo responsive polymers is defined in pure water but can also be stimulated at lower than the phase transition temperature employing electrolytes containing inorganic salts such as (NH₄)₂SO₄. Under such conditions the polymers show the typical thermo responsive phase transfer property in form of a visible clouding point. Gelatin was used to maintain this biphasic state by slowly adding water-softened gelatin sheets at a temperature above the LCST, followed by cooling to 3 °C in order to induce gelation. Examination of the gelatin-coagulate matrices by optical and electronic microscopy showed that PNIPAM and its copolymer (PNIPAM/PDMAM 1:1) are entrapped as globular spheres and clusters of spheres. In comparison pure PDMAM, even if it shows a clouding point, does not form typical LCST coagulates. With PNIPAM and the copolymer, micro globule formation is also possible with slow gelatin formation, without first provoking an LCST. In this particular case, the phase transition, or entropic demixing of the polymers respectively, are induced in this case by water absorption of the gelatin matrix.     

Effect of Topology on the Properties of Poly(N-isopropylacrylamide) in Water and in Bulk

Three macrocyclic poly(N-isopropylacrylamide)s (PNIPAM) with molecular weight (MW) ranging from 6 to 19 kg/mol were synthesized by ‘click’ ring closure of the corresponding α-azido ω-propargyl telechelic linear PNIPAMs, themselves prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide. Differential scanning calorimetry (DSC) studies revealed that both the thermal phase separation in water and the glass transition in bulk of PNIPAM were affected by polymer topology. In aqueous solution, the cyclic polymers exhibit a higher phase separation temperature and broader phase transition range than the corresponding linear counterparts. In bulk, the cyclic polymers display a higher glass transition temperature of lesser molecular weight dependence, as compared to their linear precursors.     

Effect of chain architecture on the phase transition of star and cyclic poly(N‐isopropylacrylamide) in water

The heat‐induced phase transition of aqueous solutions of Poly(N‐isopropylacrylamide) (PNIPAM) in water is examined for a four‐arm PNIPAM star (s‐PNIPAM), a cyclic PNIPAM (c‐PNIPAM), and their linear counterparts (l‐PNIPAM) in the case of polymers (1.0 g L^?1) of 12,700 g mol^?1 < M_n < 14,700 g mol^?1. Investigations by turbidity, high‐sensitivity differential scanning calorimetry (HS‐DSC), and light scattering (LS) indicate that the polymer architecture has a strong effect on the cloud point (T_c: decrease for s‐PNIPAM; increase for c‐PNIPAM), the phase transition enthalpy change (ΔH decrease for s‐PNIPAM and c‐PNIPAM), and the hydrodynamic radius of the aggregates formed above T_c (R_H: c‐PNIPAM < s‐PNIPAM < l‐PNIPAM). The properties of s‐PNIPAM are compared with those of previously reported PNIPAM star polymers (3 to 52 arms). The overall observations are described in terms of the arm molecular weight and the local chain density in the vicinity of the core of the star, by analogy with the model developed for PNIPAM brushes on nanoparticles or planar surfaces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2059–2068.     

Synthesis and thermoresponsive property of end‐functionalized poly(N‐isopropylacrylamide) with pyrenyl group

N–Isopropylacrylamide (NIPAM) was polymerized using 1‐pyrenyl 2‐chloropropionate (PyCP) as the initiator and CuCl/tris[2‐(dimethylamino)ethyl]amine (Me₆TREN) as the catalyst system. The polymerizations were performed using the feed ratio of [NIPAM]₀/[PyCP]₀/[CuCl]₀/[Me₆TREN]₀ = 50/1/1/1 in DMF/water of 13/2 at 20 °C to afford an end‐functionalized poly(N‐isopropylacrylamide) with the pyrenyl group (Py–PNIPAM). The characterization of the Py–PNIPAM using matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry provided the number–average molecular weight (M_n,MS). The lower critical solution temperature (LCST) for the liquid–solid phase transition was 21.7, 24.8, 26.5, and 29.3 °C for the Py–PNIPAMs with the M_n,MS's of 3000, 3400, 4200, and 5000, respectively; hence, the LCST was dramatically lowered with the decreasing M_n,MS. The aqueous Py–PNIPAM solution below the LCST was characterized using a static laser light scattering (SLS) measurement to determine its molar mass, M_w,SLS. The aqueous solutions of the Py–PNIPAMs with the M_n,MS's of 3000, 3400, 4200, and 5000 showed the M_w,SLS of 586,000, 386,000, 223,000, and 170,000, respectively. Thus, lowering the LCST for Py–PNIPAM should be attributable to the formation of the PNIPAM aggregates. The LCST of 21.7 °C for Py–PNIPAM with the M_n,MS of 3000 was effectively raised by adding β‐cyclodextrin (β‐CD) and reached the constant value of ～26 °C above the molar ratio of [β‐CD]/[Py–PNIPAM] = 2/1, suggesting that β‐CD formed an inclusion complex with pyrene in the chain‐end to disturb the formation of PNIPAM aggregates, thus raising the LCST. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1117–1124, 2006     

LCST behavior of copolymers of N-isopropylacrylamide and N-isopropylmethacrylamide in water

The lower critical solution temperature (LCST) behavior of copolymers of N-isopropylacrylamide (NiPA) and N-isopropylmethacrylamide (NiPMA) in water was studied as a function of the copolymer composition, using a combination of turbidity measurements and differential scanning calorimetry (DSC). The copolymers were prepared by free radical polymerization using N,N-dimethylformamide as a solvent and α,α′-azobis(isobutyronitrile) as an initiator. The copolymer composition was determined by elemental analysis. It was found that the temperature (T _c) at which the copolymer undergoes a phase transition, i.e., LCST, increases linearly with increasing the mole fraction (f _m) of NiPMA in the copolymer, within the T _c range from 32?°C (at f _m?=?0; NiPA homopolymer) to 42?°C (at f _m?=?1; NiPMA homopolymer). Also found from heating DSC thermograms were the linear dependencies of the enthalpy (ΔH) and entropy (ΔS) changes at T _c upon f _m. However, the ΔH (5.5?kJ/unit-mol) at f _m?=?1 was slightly smaller than that (5.7?kJ/unit-mol) of poly(N-n-propylacrylamide) but considerably smaller than that (7.8?kJ/unit-mol) of poly(N-n-propylmethacrylamide). The same trend was observed in the f _m dependence of ΔS. These results were discussed in terms of the structural effects of the NiPMA monomer unit on the heat-induced phase transition in water of poly(NiPA-co-NiPMA)s. It was suggested that a strong interaction of water with the amide group in the NiPMA would raise the transition temperature, but a local dehydration which occurs around the isopropyl side chain would not lead to large changes in the enthalpy and entropy at T _c.     

Polystyrene latex particles coated with crosslinked poly(N-isopropylacrylamide)

Thermoresponsive colloidal particles were prepared by seeded precipitation polymerization of N-isopropylacrylamide (NIPAM) in the presence of a crosslinking monomer, N,N-methylenebisacrylamide (MBA), using polystyrene latex particles (ca. 50 nm in diameter) as seeds in aqueous dispersion. Phase transitions of the prepared poly(N-isopropylacrylamide), PNIPAM, shells on polystyrene cores were studied in comparison to colloidal PNIPAM microgel particles, in H₂O and/or in D₂O by dynamic light scattering, microcalorimetry and by ¹H NMR spectroscopy including the measurements of spin–lattice (T1) and spin–spin (T2) relaxation times for the protons of PNIPAM. As expected, the seed particles grew in hydrodynamic size during the crosslinking polymerization of NIPAM, and a larger NIPAM to seed mass ratio in the polymerization batch led to a larger increase of particle size indicating a product coated with a thicker PNIPAM shell. Broader microcalorimetric endotherms of dehydration were observed for crosslinked PNIPAM on the solid cores compared to the PNIPAM microgels and also an increase of the transition temperature was observed. The calorimetric results were complemented by the NMR spectroscopy data of the ¹H-signal intensities upon heating in D₂O, showing that the phase transition of crosslinked PNIPAM on polystyrene core shifts towards higher temperatures when compared to the microgels, and also that the temperature range of the transition is broader.     

Hydration and phase separation of temperature-sensitive water-soluble polymers

Collapse of a poly(N-isopropylacrylamide)(PNIPAM) chain upon heating and phase diagrams of aqueous PNIPAM solutions with very flat LCST phase separation line are theoretically studied on the basis of cooperative dehydration(simultaneous dissociation of bound water molecules in a group of correlated sequence),and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods.The transition becomes sharper with the cooperativity parameterσof hydration.Reentrant coil-globule-coil transition in mixed solvent of water and methanol is also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves(LCST cononsolvency) with the mole fraction of methanol due to the competition is calculated and compared with the experimental data.Aqueous solutions of hydophobically-modified PNIPAM carrying short alkyl chains at both chain ends(telechelic PNIPAM) are theoretically and experimentally studied.The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation),and separate from the coil-globule transition line.Associated structures in the solution,such as flower micelles,mesoglobules and higher fractal assembly,are studied by USANS with theoretical modeling of the scattering function.     

Kinetics of Coil-to-Globule Transition of Dansyl-Labeled Poly(N-isopropyl-acrylamide) Chains in Aqueous Solution

The coil-to-globule transition of thermally sensitive linear poly(N-isopropylacrylamide) (PNIPAM) labeled with dansyl group is induced by 1.54 μm laser pulses (width≈10 ns). The dansyl group is used to follow the transition kinetics because its fluorescence intensity is very sensitive to its micro-environment. As the molar ratio of NIPAM monomer to dansyl group increases from 110 to 300, the effect of covalently attached dansyl fluorophores on the transition decreases. In agreement with our previous study in which we used 8-anilino-1-naphthalensulfonic acid ammonium salt free in water as a fluorescent probe, the current study reveals that the transition has two distinct stages with two characteristic times, namely, τ_fast≈0.1 ms, which can be attributed to the nucleation and formation of some "pearls" (lo-cally contracting segments) on the chain, and τ_slow≈0.5 ms, which is related to the merging and coarsening of the "pearls". τ_fast is independent of the PNIPAM chain length over a wide range (M_w=2.8×10⁶-4.2×10⁷ g/mol). On the other hand, τ_slow only slightly increases with the chain length.     

Effect of carbon chain length of monocarboxylic acids on cloud point temperature of poly(2-ethyl-2-oxazoline)

The temperature-induced phase transition of poly(2-ethyl-2-oxazoline) (PEtOx) aqueous solution under mixing with a series of small carboxylic acids has been studied by turbidity measurements and laser light scattering. It has been found that cloud point temperature (T _cp) of the PEtOx was changed to varying degrees depending upon the pH, ionic strength, molar ratio of acids to 2-ethyl-2-oxazoline unit, and carbon chain length of small carboxylic acids. Significant change in T _cp was observed in the case of hexanoic acid. At acidic pH, an increase in the molar ratio of hexanoic acid to the 2-ethyl-2-oxazoline unit gradually decreased the phase transition temperature of the polymer as compared to the T _cp of pure PEtOx. At original pH 6 (pH?>?pK _a), T _cp shifts to higher value than that of pure PEtOx for lower molar ratios and decreased later on with increasing the molar ratio. The shift in the T _cp is described based on the differences in the driving force of phase transition, including hydrogen bonding between small carboxylic acids and PEtOx polymer and hydrophobic interaction.     

Molecular weight dependence of crystal pattern transitions of poly(ethylene oxide)

Crystal patterns in ultrathin films of six poly(ethylene oxide) fractions with molecular weights from 25000 to 932000 g/mol were characterized within crystallization temperature range from 20 ℃ to 60 ℃.Labyrinthine,dendritic and faceted crystal patterns were observed in different temperature ranges,and then labyrinthine-to-dendritic and dendritic-tofaceted transition temperatures T L-D and T D-F were quantitatively identified.Their molecular weight dependences are T L-D(M w) = T L-D(∞) K L-D /M w,where T L-D(∞) = 38.2 ℃ and K L-D = 253000 ℃.g/mol and T D-F(M w) = T D-F(∞) K D-F /M w,where T D-F(∞) = 54.7 ℃ and K D-F = 27000 ℃.g/mol.Quasi two-dimensional blob models were proposed to provide empirical explanations of the molecular weight dependences.The labyrinthine-to-dendritic transition is attributed to a molecular diffusion process change from a local-diffusion to diffusion-limited-aggregation(DLA) and a polymer chain with M w ≈ 253000 g/mol within a blob can join crystals independently.The dendritic-to-faceted transition is attributed to a turnover of the pattern formation mechanism from DLA to crystallization control,and a polymer chain with a M w ≈ 27000 g/mol as an independent blob crosses to a depletion zone to join crystals.These molecular weight dependences reveal a macromolecular effect on the crystal pattern formation and selection of crystalline polymers.     

Scaling of the molecular weight‐dependent thermal volume transition of poly(N‐isopropylacrylamide)

A series of narrowly distributed poly(N‐isopropylacrylamide) (PNIPAM) with molecular weight ranging from 8 × 10⁴ to 2.3 × 10⁷ g/mol were prepared by a combination of free radical polymerization and fractional precipitation. An ultrasensitive differential scanning calorimetry was used to study the effect of molecular weight on the thermal volume transition of these PNIPAM samples. The specific heat peak of the transition temperature (T_p,0) was obtained by extrapolation to zero heating rate (HR) because of the linear dependence of the transition temperature (T_p) on the HR. The relation between T_p,0 and the degree of polymerization (N) was investigated. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1388–1393, 2010     

Aggregation behavior of pH- and thermo-responsive block copolymer protected gold nanoparticles

Two distinctive block copolymers protected gold nanoparticles (AuNPs) were prepared with poly(methylacrylic acid)-block-poly(N-isopropylacrylamide) (SH-PMAA₆₄-b-PNIPAM₃₅) and poly (N-isopropylacrylamide)-block-poly(methylacrylic acid) (SH-PNIPAM₄₀-b- PMAA₆₀) through strong gold-sulfur bonding. The hybrid NPs have a pH-responsive inner shell (or corona) and a thermo-responsive corona (or inner shell) due to different location relations of the PNIPAM and PMAA on the surface of AuNPs. Then, the aggregation behaviors, as well as the changes of optical properties, of two hybrid NPs were compared in response to both stimuli. The results showed the obvious inter-particle aggregation caused by the phase transition for hydrophobic coronal polymer. However, the particles of hydrophilic corona layer retained good dispersion and the pH-responsive or thermo-responsive characteristics of shell layer made relatively minor changes.     

The solvent quality of water for poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) in the collapsed state: Implications from single-molecule studies

Both of temperature (in water) and composition (in the water/methanol mixed solvent) can induce the coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM). The atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) has been exploited to investigate the interactions between the polymer chain and solvent at the single-molecule level. It is found that the single-chain mechanics of PNIPAM show a remarkable dependence on the two external stimuli. A confusing experimental result is that all the force-extension (F-E) curves of unfolding an individual PNIPAM globule present a feature of elastic (monotonically increasing force) stretching but not plateau (constant force) stretching predicted by theory. In this article, we clarify that the presence of the interior solvent molecules in the single-chain globule is the origin of the discrepancy between the F-E curves obtained from theory and experiment. Although both of the external stimuli do tend to lower the solvent quality for PNIPAM, water and the water/methanol mixed solvent will never be the strongly poor solvent for PNIPAM, even at the worst condition.     

Activity of enzymes immobilized on microspheres with thermosensitive hairs

Poly(N-isopropylacrylamide)s (PNIPAMs) carboxylated at one chain end or both ends were prepared by polymerization using 4,4-azobis(N,N,-cyanopentanoic acid) (V-501) as an initiator and β-mercaptopropionic acid (MPA) as a chain transfer reagent. One end group of PNIPAM carboxylated at both ends was conjugated with latex particles, and another with trypsin using carbodiimide. Differential scanning calorimetry (DSC) revealed that PNIPAM on the particles exhibited a drastic phase transition, and that the transition temperature was largely elevated when the enzyme was immobilized at the chain end. Therefore, PNIPAM on the particles showed two phase transitions because of the coexistence of the enzyme-conjugated and non-conjugated PNIPAMs. The activity of trypsin immobilized on the particles with the PNIPAM spacer showed significant temperature dependence. The apparent relative activity increased above the transition temperature of non enzyme-conjugated PNIPAM on the particles. One of the reasons for this is that the diffusion of the substrate changed discontinuously around the transition temperature. Therefore, the temperature dependence of the enzymatic activity was significantly affected by the molecular size of the substrates. The enzymatic activity was also influenced by the surface density of trypsin and PNIPAM on the particle, and the molecular weight of the PNIPAM spacer.     

Apparent and partial molar volumes of long-chain alkyldimethylbenzylammonium chlorides and bromides in aqueous solutions at T=15 °C and T=25 °C

Density measurements of dodecyl- (C₁₂DBACl), tetradecyl- (C₁₄DBACl), hexadecyldimethylbenzylammonium chloride (C₁₆DBACl) and of decyl- (C₁₀DBABr) and dodecyldimethylbenzylammonium bromide (C₁₂DBABr) in aqueous solutions at T=15 °C and T=25 °C have been carried out. From these results, apparent and partial molar volumes were calculated. Positive deviations from the Debye-Hückel limiting law provide evidence for limited association at concentrations below the critical micelle concentration. The change of the apparent molar volume upon micellization was calculated. The relevant parameters have been presented in function of the alkyl chain length. Apparent molar volumes of the present compounds in the micellar phase, V_φ^m, and the change upon micellization, ΔV_φ^m, have been discussed in terms of temperature and type of counterion.     

Differential scanning calorimetry study of Ge1−xSnxSe2.5 glasses

The glass-forming tendency and specific heat in ice cold water-quenched Ge_1?xSn_xSe_2.5 glassy alloys with 0H _f, the heat ΔH _c associated with the crystallization of an amorphous phase and the glass transition temperatureT _g were deduced from the DSC curves. The composition dependence of glass forming ability,T _g and crystallization behavior has been discussed.     

Magnetocaloric effect and the heat capacity of ferrimagnetic nanosystems in magnetic fluids

The specific heat capacity of a magnetite-based magnetic fluid and changes in the magnetic part of the molar heat capacity of its magnetic phase in magnetic fields of 0–0.7 T were determined calorimetrically over the temperature range 288–353 K. The temperature dependence of changes in the magnetic part of entropy in an applied magnetic field was calculated. It was found that the field dependence of heat capacity had a maximum in fields of 0.3–0.4 T, and the temperature dependences of changes in the magnetic part of heat capacity ΔC _p (H) and entropy ΔS _m(H) had maxima at the magnetic phase transition temperature.     

Synthesis of thermoresponsive polystyrene-based comb-type grafted poly( N -isopropylacrylamide)

Narrowly distributed polystyrene-g-p(N-isopropylacrylamide) (PSt-g-PNIPAM) was prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide using the brominated polystyrene as macroinitiator and CuCl combined with hexamethyltriethylenetetramine as catalyst. Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy confirmed the structure of PSt-g-PNIPAM. The gel permeation chromatography (GPC) showed that the graft copolymer had a single distribution peak with molecular weight, M _n (g/mol) of 19815 g/mol (using polystyrene as the standard). Differential scanning calorimetry (DSC) revealed that due to both effects of hydrophobic isopropyl groups and hydrogen bonds in the amide group, the glass transition temperature (T _g) of PSt-g-PNIPAM enhanced 16.0°C compared to the T _g of the polystyrene.     

Direct imaging of temperature-sensitive core-shell latexes by cryogenic transmission electron microscopy

We present a comprehensive investigation of the volume transition in thermosensitive core-shell particles. The particles consist of a solid core of poly (styrene) (radius: 52 nm) onto which a network of crosslinked poly(N-isopropylacrylamide) (PNIPAM) is affixed. The degree of crosslinking of the PNIPAM shell effected by the crosslinker N,N ^′-methylenebisacrylamide was varied between 1.25 and 5 mol%. Immersed in water, the shell of these particles is swollen at low temperatures. Raising the temperature above 32°C leads to a volume transition within the shell. Cryogenic transmission electron microscopy (Cryo-TEM) and dynamic light scattering (DLS) have been used to investigate the structure and swelling of the particles. The Cryo-TEM micrographs directly show inhomogeneities of the network. Moreover, a buckling of the shell from the core particle is evident. This buckling increases with decreasing degree of crosslinking. A comparison of the overall size of the particles determined by DLS and Cryo-TEM demonstrates that the hydrodynamic radius provides a valid measure for the size of the particles. The phase transition within the network measured by DLS can be described by the Flory–Rehner theory. It is shown that this model captures the main features of the volume transition within the core-shell particles including the dependence of the phase transition on the degree of crosslinking. All dispersions crystallize at volume fractions above 0.5. The resulting phase diagram is identical to the phase behavior of hard spheres within the limits of error. This demonstrates that the core-shell microgels can be treated as hard spheres up to volume fractions of at least 0.55.     

Synthesis and self-assembly behavior of thermoresponsive triblock copolymer

Block copolymers comprising thermosensitive poly(N-isopropylacrylamide) (PNIPAM) and hydrophobic poly(n-butyl acrylate) (PBA) blocks, were synthesized using the reversible addition-fragmentation chain transfer polymerization (RAFT), their thermosensitive behavior was studied by ultraviolet spectrophotometer (UV) and dynamic light scattering (DLS). The lower critical solution temperature (LCST) was strongly correlated to the hydrophobic/hydrophilic ratio of the copolymers. Their micellization and self-assembly behavior in dilute aqueous solution were studied by surface tension (SFT), DLS and TEM. The resulting block copolymers reversibly formed or deformed micellar assemblies during their LCSTs. The critical micelle concentration (CMC) was controlled by the composition of PBA and PNIPAM, indicating the successful formation of the block copolymers.