Aggregation Behavior of Nonsymmetrically End-Capped Thermoresponsive Block Copolymers in Aqueous Solutions: Between Polymer Coils and Micellar States

Abstract

The thermosensitive aggregation behavior in an aqueous solution of a library of amphiphilic BAB* copolymers is studied, where "A" represents a long permanently hydrophilic poly(N,N-dimethylacrylamide) (pDMAm) block, "B" represents a permanently hydrophobic end with an n-dodecyl chain, and "B*" represents a thermoresponsive (TR) block featuring a lower critical solution temperature (LCST). Four polyacrylamides are employed for B*, namely, poly(N-n-propylacrylamide) (pNPAm), poly(Nisopropylacrylamide) (pNiPAm), poly(N,N-diethylacrylamide) (pDEAm), and poly(N-acryloylpyrrolidine) (pNAP), which differ with respect to the hydrophilicity of their amide side chains and LCST behavior. While blocks A and B were kept constant, the lengths of the TR blocks were varied systematically. These amphiphilic copolymers were studied as a function of concentration and temperature via light and neutron scattering (static light scattering (SLS), dynamic light scattering (DLS), small-angle neutron scattering (SANS)). For sufficiently long pNiPAM and pDEAm blocks (DP n > 40), a pronounced hydrophobic effect at temperatures above the LCST transition results in well-structured, ordered aggregates. Thus, the aggregation can be controlled by the choice and length of the TR block, thereby elucidating a so far hardly explored class of temperature-sensitive polymeric amphiphiles.

Rheological Control of Aqueous Dispersions by Thermoresponsive BAB* Copolymers of Different Architectures

“…Apparently, the effective volume fraction of the aggregates has already become so high that the attractive bridging effect introduced by forming hydrophobic domains of the TR-block above the LCST is counterbalanced by the dense packing (see Scheme ). Similar structural behavior has been reported before for such a type of copolymer BAB* …”

“…To obtain the distribution of apparent hydrodynamic radii R h app , D was converted via the Stokes–Einstein equation, R h app = k B T /(6πη D ), where k B is the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. Further details can be found elsewhere …”

“…Monomer N -isopropylacrylamide (NiPAm, 97%, Merck/Germany) was recrystallized from n -heptane prior to use. Monomer N -propylacrylamide (NPAm) was prepared as described elsewhere . The synthesis of chain transfer agent 4-(dodecylcarbamoyl)­benzyl-3-(trimethylsilyl)­propyl)-carbonotrithioate ( CTA-L ) was reported before .…”

“…In this general context, we have previously explored a set of amphiphilic copolymers of the unusual BAB* type, i.e., with an unsymmetrical architecture . Here, B is a permanently hydrophobic segment, A is a permanently hydrophilic block, and B* is a thermoresponsive (TR) block that can be triggered to switch from hydrophilic to hydrophobic in the temperature range of 20–60 °C.…”

“…For TR blocks B*, poly­( N -propylacrylamide) (pNPAm, LCST ≈ 22 °C for high molar masses), poly­( N , N diethylacrylamide) (pDEAm, LCST ≈ 30 °C for high molar masses), and poly­( N -isopropylacrylamide) (pNiPAm, LCST ≈ 32 °C for high molar masses) were selected, with increasing LCST for the pure homopolymers. Although the basic temperature window is defined by the monomer used, it is understood that the precise values of the respective LCSTs depend also on the molar mass of the polymer block studied, decreasing asymptotically with its increasing length …”