Better solvents generally mean more swelling
Solvent quality is a measure of how favorably the liquid interacts with the polymer chains. A good solvent surrounds the chains comfortably and lets the network expand; a poor solvent does the opposite. In a study of chemically cross-linked poly(vinyl acetate) gels in isopropyl alcohol, swelling and deswelling tracked temperature relative to the theta temperature, the point at which the polymer's attraction to itself and to the solvent balance out. The gels' swelling followed a universal scaling pattern of the same general form used for free polymer chains in solution [1].
That result matters because it ties gel swelling to a property of the surrounding liquid rather than to the cross-links alone. The theta temperature of the gels was close to that of the corresponding uncross-linked polymer solution, so solvent quality could be defined in the same way for both [1]. In practical terms, moving a gel from a poorer to a better solvent is like loosening a cramped space: the network can take in more liquid and expand.
Two poor solvents can combine to swell a polymer
The simple rule has a striking exception. When two liquids that are each poor solvents for a polymer are mixed, the polymer can swell at intermediate mixing ratios, producing a collapse–swelling–collapse pattern as the mixture changes [3]. Simulations and theoretical arguments attribute this to depletion interactions—entropically driven effects that arise when the mixture's components are purely repulsive—reaching a delicate balance at constant pressure [3].
This is a useful reminder that 'solvent quality' is not a single dial. A mixture can behave unlike either of its ingredients, so the swelling of a polymer in a blended solvent cannot be predicted just by averaging the quality of the two pure liquids. The proposed depletion mechanism is a theoretical explanation for the pattern, not a settled account of every mixture.
The polymer's own structure changes which solvent swells it
Solvent quality also depends on what the polymer is made of and how those pieces are arranged. In copolymer brushes, water is a good solvent for one component (quaternized DMA) but not for the other (DiPA), while isopropanol reverses that preference. In water, adding more DiPA reduced swelling for both block and statistical brushes. In isopropanol, the responses diverged: block copolymer brushes swelled 9–37% less than statistical brushes of the same overall composition [5].
The lesson is that the spatial distribution of solvophobic and solvophilic units—not just their relative abundance—governs how a brush responds to a given solvent [5]. A brush can therefore be tuned to swell in one liquid and stay compact in another, which is relevant to applications such as nanoparticle transport and sensing [5].
Swelling shows up as moving fronts and thickness changes
In practical materials, solvent-driven swelling is often observed as a moving boundary. Xanthan-based polymer matrix tablets hydrated in water showed well-resolved fronts of erosion, swelling, and penetration, and simulations that modeled the tablet as interacting chains with embedded drug particles reproduced the hydration process [2]. This matters for drug delivery, where the rate at which water penetrates and the matrix swells helps set how the drug is released [2].
Thin conducting-polymer films show the same principle in a different setting. For cross-linked PEDOT:PSS films, thickness changes across a range of non-aqueous solvents were described well by a traditional Flory–Huggins model, and electrochemical oxidation and reduction also produced dramatic thickness changes, with films expanding on reduction and shrinking on oxidation [4]. Including a voltage dependence in the mixing enthalpy let the same Flory–Huggins approach describe the electrochemically driven swelling [4].
Sources used in this answer
Influence of solvent quality on the swelling and shear modulus of polymer gels chemically cross-linked in solution
Swelling and deswelling of poly(vinyl acetate) gels in isopropyl alcohol followed a universal scaling equation of the same general form as renormalization-group theory for flexible linear polymer chains, with solvent quality defined by temperature relative to the theta temperature.
The Effect of Polymer–Solvent Interaction on the Swelling of Polymer Matrix Tablets: A Magnetic Resonance Microscopy Study Complemented by Bond Fluctuation Model Simulations
Xanthan-based polymer matrix tablets hydrated with well-resolved erosion, swelling, and penetration fronts, and bond-fluctuation simulations modeled the hydration process as interacting chains with embedded drug particles.
Depleted Depletion Drives Polymer Swelling in Poor Solvent Mixtures
Mixtures of two miscible poor solvents can produce a collapse–swelling–collapse pattern in a polymer, attributed to a delicate balance of entropically driven depletion interactions at constant pressure.
Electrochemical and Solvent-Driven Swelling in a Conducting Polymer Film
Swelling of cross-linked PEDOT:PSS films in non-aqueous solvents was described by a Flory–Huggins model, and electrochemical reduction increased film thickness while oxidation decreased it.
Polymer Microstructure Directs the Solvent-Selective Swelling Response of Polymer Brushes
In copolymer brushes, the spatial distribution of solvophobic and solvophilic units governed solvent-selective swelling; in isopropanol, block brushes swelled 9–37% less than compositionally equivalent statistical brushes.