💡 In This Article
- What storage modulus (G′) and loss modulus (G″) mean in rheology
- How Rheometer and DMA measurements differ—and why their results can look surprisingly similar
- How a Rheometer can be used to evaluate the glass-transition behavior of a cured epoxy adhesive
In the previous article, we looked at the gel point — the stage of curing where an epoxy adhesive starts behaving more like a solid than a flowing liquid. But curing doesn’t end at the gel point. Even after the material has formed a solid network, its mechanical behavior keeps changing as crosslinking continues, and eventually a different question comes up: how does the fully cured adhesive respond to temperature?..
This is where things get interesting. If you’ve worked with a DMA, you’re probably already familiar with parameters like storage modulus (E′), loss modulus (E″), and tan δ. Surprisingly, very similar concepts show up in rheometer measurements too, expressed there as G′ and G″.. At first that can be confusing — a rheometer is usually thought of as an instrument for measuring viscosity and flow, while DMA is typically associated with the mechanical properties of solid materials. So why do these two instruments produce such similar-looking data?
The answer is viscoelasticity. The two instruments don’t measure exactly the same physical quantity, and they don’t deform a material the same way, but both are examining how a material responds to deformation when elastic and viscous behavior are both present. In this article, we’ll look at why these seemingly different techniques can reveal remarkably similar information, and how a rheometer can be used to evaluate the temperature-dependent behavior of a cured epoxy adhesive.
Two Instruments, One Fundamental Question
A rheometer and a dynamic mechanical analyzer aren’t interchangeable instruments. A DMA typically characterizes solid materials by applying a controlled mechanical deformation — tension or bending, for instance — and measuring the resulting mechanical response. A rotational rheometer, on the other hand, is usually associated with liquids and flow: it applies controlled shear through a rotating geometry and measures the resulting torque.
At first glance there’s little reason to expect similar-looking data from the two. But both instruments are ultimately asking the same fundamental question: when a material is deformed, how much of the applied energy does it store, and how much does it dissipate? That’s the essence of viscoelasticity. A polymer is rarely a perfectly elastic solid or a perfectly viscous liquid — its response usually sits somewhere between the two, and that balance shifts with temperature, frequency, deformation, molecular structure, and the material’s physical state.
What Are G′ and G″?
In an oscillatory rheometer test, the material is subjected to a small, cyclic shear deformation. Part of the mechanical energy gets stored within the material and returned during the deformation cycle — that’s the storage modulus, G′. Another portion dissipates through internal molecular and structural processes — the loss modulus, G″. Put simply, G′ tells us how strongly the material behaves like an elastic structure, and G″ tells us how strongly it behaves like a viscous, energy-dissipating material. The ratio of the two is commonly expressed as tan δ = G″/G′, giving a convenient measure of the balance between elastic and viscous behavior.
These parameters become especially informative under a temperature sweep. As temperature rises, a polymer network can gradually lose rigidity and grow more mobile, and the changes in G′, G″, and tan δ let us observe that transition from a different physical angle than a simple viscosity measurement.
So Where Does DMA Come In?
DMA uses a different deformation mode, but the basic idea is remarkably similar. In a tensile DMA measurement, for example, a specimen is repeatedly stretched and released, and the mechanical response again splits into an energy-storing component and an energy-dissipating component. DMA reports E′ (storage modulus), E″ (loss modulus), and tan δ (loss factor).
The key distinction is the type of deformation: DMA modulus is generally expressed as E — tensile or flexural modulus — while a rotational rheometer expresses modulus as G, the shear modulus. So E′ and G′ aren’t the same measurement, and neither are E″ and G″ — the deformation mode, specimen geometry, and mechanical boundary conditions all differ. What the two share is the physical concept behind the measurement: both techniques separate the material’s response into elastic and dissipative components, which is why their temperature-dependent behavior can sometimes look remarkably similar.
A Development Note: I Learned This From a Practical Problem
This connection wasn’t just a textbook concept in my own work. The lab I worked in didn’t have a DMA at the time, which created a real practical problem — we still needed to understand how epoxy materials responded to temperature, particularly around the glass transition. So we used the rheometer instead.
A thin epoxy specimen was placed between disposable plates and run in oscillation mode while temperature increased, tracking G′ and G″ throughout the sweep. When we later compared the results against DMA data, the curves weren’t identical — and they shouldn’t have been — but the overall temperature-dependent transition was remarkably similar. The material grew progressively softer as it approached the transition region, and the relative contributions of elastic and viscous behavior shifted significantly. That experience drove home a point I’ve come back to often: different instruments can observe the same fundamental material transition through different physical responses. That’s the connection between rheology and DMA.
Can a Rheometer Be Used to Evaluate Tg?
Yes, with an important qualification. A rheometer doesn’t measure Tg through exactly the same physical principle as DSC or DMA, but it can be used to examine a cured polymer’s glass-transition behavior through its shear viscoelastic response. For a cured epoxy adhesive, the test typically runs in three steps.
Step 1 — Prepare and cure the specimen. A thin layer of uncured epoxy is placed between disposable plates and cured under a controlled temperature program, with the goal of building the cured polymer network.
Step 2 — Cool the specimen. Once curing is complete, the specimen is cooled back to a defined starting temperature, establishing a consistent initial condition for the temperature sweep that follows.
Step 3 — Perform an oscillatory temperature sweep. The cured specimen is reheated while the rheometer applies a small oscillatory shear deformation, tracking G′, G″, and tan δ against temperature. The resulting curves show how the cured epoxy’s mechanical character shifts as it approaches and moves through the glass-transition region.
This is fundamentally different from using a rheometer to measure the viscosity of an uncured adhesive — the material state, the test objective, and the information we get out of it have all changed.
Why the Disposable Geometry Matters
There’s also a practical reason this kind of experiment is often run with disposable plates: once epoxy has cured, it can bond strongly to the geometry, and removing a cured specimen without damaging the geometry — or contaminating it for the next test — can be genuinely difficult. A disposable geometry lets the whole sequence run on the same specimen (uncured adhesive → cure → cool → reheat → oscillatory measurement), and once the experiment finishes, the specimen and geometry can simply be discarded. For formulation labs working across many epoxy systems, that’s a real practical advantage.
Tg Depends on What You Measure
This is one of the most useful ways to think about thermal analysis. The glass transition is a single physical phenomenon, but different analytical techniques observe it through different measurable responses: DSC detects a change in heat capacity, TMA observes a change in dimensional behavior, DMA detects a change in mechanical response, and a rheometer observes changes in shear viscoelasticity. The phenomenon is the same; the measured physical response differs. That’s why Tg values from different analytical techniques don’t necessarily line up exactly — and a mismatch doesn’t automatically mean one measurement is wrong. The techniques are simply looking at the transition from different perspectives under different experimental conditions.
For an epoxy formulator, that distinction matters. Rather than asking which instrument gives the “correct” Tg, it’s usually more useful to ask what aspect of the material’s behavior you’re actually trying to understand. DSC may be the right call for thermal transitions, TMA for dimensional stability, and DMA for mechanical performance through the glass transition — and if the material is already being studied rheologically, a rheometer can offer another perspective on the same transition through shear viscoelasticity.
One Rheometer, Two Very Different Jobs
A rheometer, in other words, can tell us far more than how easily an epoxy adhesive flows. In the first article of this series, we looked at the rheometer from a processing perspective — how viscosity changes with shear rate and what that means for dispensing and coating. In the second, we moved into the curing process and examined the gel point, where the material shifts from predominantly viscous behavior toward a solid-like network. Here, we’ve taken one more step and looked at the cured adhesive from yet another angle: how its viscoelastic behavior changes with temperature.
The same instrument, in other words, can follow very different stages of an epoxy adhesive’s life — from flow, through gelation, to the temperature-dependent behavior of the cured network. That’s one of the reasons I find rheology so useful in adhesive development. It’s not simply a technique for measuring viscosity — with the right test method, a rheometer can offer a window into the material’s behavior throughout the entire curing process and even after it has become a solid. For an adhesive formulator, understanding that behavior is often worth more than any single number.
